We dropped MySQL support and a lot of mysql specific code has been
removed in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29608.

This comes in from the other direction and removes any `if postgresql?`
branches.

Updates changed method names and fixes spec failures

Enables frozens string for the following:

* lib/gitlab/conflict/**/*.rb
* lib/gitlab/cross_project_access/**/*.rb
* lib/gitlab/cycle_analytics/**/*.rb
* lib/gitlab/data_builder/**/*.rb
* lib/gitlab/database/**/*.rb
* lib/gitlab/dependency_linker/**/*.rb
* lib/gitlab/diff/**/*.rb
* lib/gitlab/downtime_check/**/*.rb
* lib/gitlab/email/**/*.rb
* lib/gitlab/etag_caching/**/*.rb

Partially addresses gitlab-org/gitlab-ce#47424.

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 fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/38634.

This ensures we can check if the user has TRIGGER permissions without
querying restricted tables. Thanks to Steve Norman
(https://gitlab.com/stevenorman) for helping out with this merge
request.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/38372

This adds a bunch of checks to migrations that may create or drop
triggers. Dropping triggers/functions is done using "IF EXISTS" so we
don't throw an error if the object in question has already been dropped.
We now also raise a custom error (message) when the user does not have
TRIGGER privileges. This should prevent the schema from entering an
inconsistent state while also providing the user with enough information
on how to solve the problem.

The recommendation of using SUPERUSER permissions is a bit extreme but
we require this anyway (Omnibus also configures users with this
permission).

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/36633