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

This gem allows Sidekiq jobs to be throttled. Unfortunately, it has a
side-effect: when we haven't enabled job throttling, it will still hit Redis a
lot (and miss, because nothing is configured).

As this setting already required a restart, ensure that the library is only
required when it's enabled.

Due to a missing `on_delete: :cascade`, users would hit the error that
looked like:

```
PG::ForeignKeyViolation: ERROR: update or delete on table "protected_tags"
violates foreign key constraint "fk_rails_f7dfda8c51" on table
"protected_tag_create_access_levels" DETAIL: Key (id)=(1385) is still
referenced from table "protected_tag_create_access_levels". : DELETE FROM
"protected_tags" WHERE "protected_tags"."id" = 1385
```

Closes #36013

- Adds a new `ProjectMovedError` class to encapsulate that error
  condition. Inherits from `NotFoundError` so existing rescues should
  continue to work.
- Separating that condition out of `NotFoundError` allowed us to
  simplify the `raise_not_found` helper and avoid repeating the literal
  string.
- Spec makes use of `ERROR_MESSAGES` hash to avoid repeating literal
  error message strings.

Guess the modes based on the following:

1. If the file didn't exist, it's zero.
2. If the diff contains 'Subproject commit', it might be a submodule, so 0600.
3. Otherwise, it's 0644.

This isn't perfect, but it doesn't have to be - it won't change file modes in
the repository.

This API was mainly for internal usage, and has been moved to the
general API: APIv4. The endpoints have been deprecated since 9.0, and
won't see 10.0. :)

And the link to the last page cannot be `page=0`.

Backports EE mirror stuck handling feature (https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2628) to CE

Main feature was the deprication of the Hashie stuff, so the access by
calling keys as method is gone now.

The `Rails` object was not always available in all tasks that require
Redis access, such as `mail_room`, so the constant pointing to the
configuration path was never defined, but we still attempted to access
it in `config_file_name`, resulting in a `NameError` exception.

Further, there was no benefit to defining these paths in a constant to
begin with -- they're only accessed in one place, and it was within the
class where they were being defined. We can just provide them at
run-time instead.

Further _still_, we were calling `File.expand_path` on the absolute path
returned by `Rails.root.join`, which was rather pointless.

Related to !13108. Mostly this is just running the rake task and
changing the task a bit to catch cases like the project already existing
or so. The rake task moves archives to the vendor/project_template
directory, which are checked in too.

An upcoming update to rubocop-gitlab-security added additional
violations.

The number of forks of a project doesn't change very frequently and
running a COUNT(*) every time this information is requested can be quite
expensive. We also end up running such a COUNT(*) query at least twice
on the homepage of a project.

By caching this data and refreshing it when necessary we can reduce
project homepage loading times by around 60 milliseconds (based on the
timings of https://gitlab.com/gitlab-org/gitlab-ce).

setting of the gpg home directory is not thread safe, as the directoy
gets stored on the class.

if multiple threads change the directory at the same time, one of the
threads will be working in the wrong directory.

Update the markdown unconditionally since the move might have been
done before, but the markdown not updated.

Also marks a few things as uncovered, and removes an unused class.