- Avoid N+1 queries for authors and comment counts
- Avoid an additional snippet existence query

Adding new `AddAwardEmoji`, `RemoveAwardEmoji` and `ToggleAwardEmoji`
GraphQL mutations.

Adding new `#authorized_find_with_pre_checks!` and (unused, but for
completeness `#authorized_find_with_post_checks!`) authorization
methods. These allow us to perform an authorized find, and run our own
additional checks before or after the authorization runs.

https://gitlab.com/gitlab-org/gitlab-ce/issues/62826

Signed-off-by: Rémy Coutable <remy@rymai.me>

This completely rewrites the SnippetsFinder class from the ground up in
order to improve its performance. The old code was beyond salvaging. It
was complex, included various Rails 5 workarounds, comments that
shouldn't be necessary, and most important of all: it produced a really
poorly performing database query.

As a result, I opted for rewriting the finder from scratch, instead of
trying to patch the existing code. Instead of trying to reuse as many
existing methods as possible, I opted for defining new methods
specifically meant for the SnippetsFinder. This requires some extra code
here and there, but allows us to have much more control over the
resulting SQL queries. It is these changes that then allow us to produce
a _much_ more efficient query.

To illustrate how bad the old query was, we will use my own snippets as
an example. Currently I have 52 snippets, most of which are global ones.
To retrieve these, you would run the following Ruby code:

    user = User.find_by(username: 'yorickpeterse')

    SnippetsFinder.new(user, author: user).execute

On GitLab.com the resulting query will take between 10 and 15 seconds to
run, producing the query plan found at
https://explain.depesz.com/s/Y5IX. Apart from the long execution time,
the total number of buffers (the sum of all shared hits) is around 185
GB, though the real number is probably (hopefully) much lower as I doubt
simply summing these numbers produces the true total number of buffers
used.

The new query's plan can be found at https://explain.depesz.com/s/wHdN,
and this query takes between 10 and 100-ish milliseconds to run. The
total number of buffers used is only about 30 MB.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/52639

It's possible that user pastes accidentally also unsubscribe link
which is included in footer of notification emails. This unsubscribe
link contains personal token which attacker then use to act as the
original user (e.g. for sending comments under his/her identity).

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

The status is shown for
- The author of a commit when viewing a commit
- Notes on a commit (regular/diff)
- The user that triggered a pipeline when viewing a pipeline
- The author of a merge request when viewing a merge request
- The author of notes on a merge request (regular/diff)
- The author of an issue when viewing an issue
- The author of notes on an issue
- The author of a snippet when viewing a snippet
- The author of notes on a snippet
- A user's profile page
- The list of members of a group/user

Partially addresses #47424.

* adds basic discussions API for issues and snippets
* reorganizes notes specs (so same tests can be used for all noteable types - issues, MRs, snippets)