These are not required because MySQL is not
supported anymore

CE-port of 10546-fix-epic-depth-validation

- we now use the hierarchy class also for epics
- also rename supports_nested_groups? into supports_nested_objects?
  - move it to a concern

- Rename ordered_group_clusters_for_project ->
ancestor_clusters_for_clusterable
- Improve name of order option. It makes much more sense to have `hierarchy_order: :asc`
and `hierarchy_order: :desc`
- Allow ancestor_clusters_for_clusterable for group
- Re-use code already present in Project

Look for matching clusters starting from the closest ancestor, then go
up the ancestor tree.

Then use Ruby to get clusters for each group in order. Not that
efficient, considering we will doing up to `NUMBER_OF_ANCESTORS_ALLOWED`
number of queries, but it's a finite number

Explicitly order query by depth

This allows us to control ordering explicitly and also to reverse the
order which is useful to allow us to be consistent with
Clusters::Cluster.on_environment (EE) which does reverse ordering.

Puts querying group clusters behind Feature Flag. Just in case we have
issues with performance, we can easily disable this

This commit adds the module `FromUnion`, which provides the class method
`from_union`. This simplifies the process of selecting data from the
result of a UNION, and reduces the likelihood of making mistakes. As a
result, instead of this:

    union = Gitlab::SQL::Union.new([foo, bar])

    Foo.from("(#{union.to_sql}) #{Foo.table_name}")

We can now write this instead:

    Foo.from_union([foo, bar])

This commit also includes some changes to make this new setup work
properly. For example, a bug in Rails 4
(https://github.com/rails/rails/issues/24193) would break the use of
`from("sub-query-here").includes(:relation)` in certain cases. There was
also a CI query which appeared to repeat a lot of conditions from an
outer query on an inner query, which isn't necessary.

Finally, we include a RuboCop cop to ensure developers use this new
module, instead of using Gitlab::SQL::Union directly.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/51307

This whitelists all existing offenses for the various CodeReuse cops, of
which most are triggered by the CodeReuse/ActiveRecord cop.

Passing a parent_id will limit ancestors upto the specified parent if
it is found.

Using `ancestors` and `descendants` the `base` relation will not be included

Prior to this commit running
Namespace#force_share_with_group_lock_on_descendants would result in
updating _all_ namespaces in the namespaces table, not just the
descendants. This is the result of ActiveRecord::Relation#update_all not
taking into account the CTE. To work around this we use the CTE query as
a sub-query instead of directly calling #update_all.

To prevent this from happening the relations returned by
Gitlab::GroupHierarchy are now marked as read-only, resulting in an
error being raised when methods such as #update_all are used.

Fortunately on GitLab.com our statement timeouts appear to have
prevented this query from actually doing any damage other than causing
a very large amount of dead tuples.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/37916

When a user is authorized to a group, they are also authorized to see all the
ancestor groups and descendant groups.

When a user is authorized to a project, they are authorized to see all the
ancestor groups too.

Closes #32135

See merge request !11764

This hides/disables some UI elements and API parameters related to
nested groups when MySQL is used, since nested groups are not supported
for MySQL.

This commit introduces the usage of Common Table Expressions (CTEs) to
efficiently retrieve nested group hierarchies, without having to rely on
the "routes" table (which is an _incredibly_ inefficient way of getting
the data). This requires a patch to ActiveRecord (found in the added
initializer) to work properly as ActiveRecord doesn't support WITH
statements properly out of the box.

Unfortunately MySQL provides no efficient way of getting nested groups.
For example, the old routes setup could easily take 5-10 seconds
depending on the amount of "routes" in a database. Providing vastly
different logic for both MySQL and PostgreSQL will negatively impact the
development process. Because of this the various nested groups related
methods return empty relations when used in combination with MySQL.

For project authorizations the logic is split up into two classes:

* Gitlab::ProjectAuthorizations::WithNestedGroups
* Gitlab::ProjectAuthorizations::WithoutNestedGroups

Both classes get the fresh project authorizations (= as they should be
in the "project_authorizations" table), including nested groups if
PostgreSQL is used. The logic of these two classes is quite different
apart from their public interface. This complicates development a bit,
but unfortunately there is no way around this.

This commit also introduces Gitlab::GroupHierarchy. This class can be
used to get the ancestors and descendants of a base relation, or both by
using a UNION. This in turn is used by methods such as:

* Namespace#ancestors
* Namespace#descendants
* User#all_expanded_groups

Again this class relies on CTEs and thus only works on PostgreSQL. The
Namespace methods will return an empty relation when MySQL is used,
while User#all_expanded_groups will return only the groups a user is a
direct member of.

Performance wise the impact is quite large. For example, on GitLab.com
Namespace#descendants used to take around 580 ms to retrieve data for a
particular user. Using CTEs we are able to reduce this down to roughly 1
millisecond, returning the exact same data.

== On The Fly Refreshing

Refreshing of authorizations on the fly (= when
users.authorized_projects_populated was not set) is removed with this
commit. This simplifies the code, and ensures any queries used for
authorizations are not mutated because they are executed in a Rails
scope (e.g. Project.visible_to_user).

This commit includes a migration to schedule refreshing authorizations
for all users, ensuring all of them have their authorizations in place.
Said migration schedules users in batches of 5000, with 5 minutes
between every batch to smear the load around a bit.

== Spec Changes

This commit also introduces some changes to various specs. For example,
some specs for ProjectTeam assumed that creating a personal project
would _not_ lead to the owner having access, which is incorrect. Because
we also no longer refresh authorizations on the fly for new users some
code had to be added to the "empty_project" factory. This chunk of code
ensures that the owner's permissions are refreshed after creating the
project, something that is normally done in Projects::CreateService.