- `Gitlab::Shell.fetch_remote` now takes a `Gitlab::Git::Repository` instead

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

Since we don't support nested groups on MySQL, raise an error
explaining that on import instead of trying anyway.

For some old merge requests, we don't have enough information to figure out the
old blob and the new blob for the file. This means that we can't highlight the
diff correctly, but we can still display it without highlighting.

Users of project mirrors would see that the number of branches did not
match the number in the branch dropdown because remote branches were
counted when Rugged was in use. With Gitaly, only local branches
are counted.

Closes #36934

Support simple string LDAP attribute specifications, and search for name rather than username attributes

- Allow imports into nested groups
- Make sure it sets the correct visibility level when creating new
  groups

While doing this, I moved the import into a testable class, that made
it easier to improve.