Remove unnecessary require_relative calls from service classes.

See merge request !7601

Rails by default use autoload for all dirs from app folder.
require_relative not needed. See ActiveSupport::Dependencies.autoload_paths

Fix errors happening when source branch of merge request is removed and then restored

See merge request !7568

Smarter cache invalidation

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/23550

See merge request !7360

This refactors repository caching so it's possible to selectively
refresh certain caches, instead of just expiring and refreshing
everything.

To allow this the various methods that were cached (e.g. "tag_count" and
"readme") use a similar pattern that makes expiring and refreshing
their data much easier.

In this new setup caches are refreshed as follows:

1. After a commit (but before running ProjectCacheWorker) we expire some
   basic caches such as the commit count and repository size.

2. ProjectCacheWorker will recalculate the commit count, repository
   size, then refresh a specific set of caches based on the list of
   files changed in a push payload.

This requires a bunch of changes to the various methods that may be
cached. For one, data should not be cached if a branch used or the
entire repository does not exist. To prevent all these methods from
handling this manually this is taken care of in
Repository#cache_method_output. Some methods still manually check for
the existence of a repository but this result is also cached.

With selective flushing implemented ProjectCacheWorker no longer uses an
exclusive lease for all of its work. Instead this worker only uses a
lease to limit the number of times the repository size is updated as
this is a fairly expensive operation.

Do not create TODO when build is allowed to fail

## What does this MR do?

Do not create a TODO when build that failed is allowed to fail.

## Does this MR meet the acceptance criteria?

- [x] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- Tests
  - [x] Added for this feature/bug
  - [ ] All builds are passing

## What are the relevant issue numbers?

Closes #22280

See merge request !7618

Backport some changes done from Time Tracking feature in EE.

See merge request !7604

This reverts commit f913170e2f76ef44800f0272cb7fb40b9d6709ee.

Merge request !7163 caused an error message to appear on the Merge
Request form when it was first loaded by the user. This commit fixes
that regression bug.