Fix race conditions for AuthorizedProjectsWorker

Closes #26194 and #26310

See merge request !8701

Reduce DB-load for build-queues by storing last_update in Redis

See merge request !8084

Use ReactiveCaching to update external CI status asynchronously

See https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/2055

Addresses: Issue #13810

1. Adds a last_used_at attribute to the Key table/model
2. Update a key's last_used_at whenever it gets used
3. Display how long ago an ssh key was last used

This adds counters for build artifacts and LFS objects, and moves
the preexisting repository_size and commit_count from the projects
table into a new project_statistics table.

The counters are displayed in the administration area for projects
and groups, and also available through the API for admins (on */all)
and normal users (on */owned)

The statistics are updated through ProjectCacheWorker, which can now
do more granular updates with the new :statistics argument.

Smarter refreshing of authorized projects

This MR reworks the way `User#refresh_authorized_projects`, resulting in a much more efficient process leading to fewer dead tuples.

Related issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/25257

See merge request !7956

Prior to this commit the refreshing of authorized projects was done in
two steps:

1. Remove existing authorizations
2. Insert a new list of all authorizations

This can lead to a high amount of dead tuples as every time all rows are
being replaced. For example, if a user with 100 authorizations is given
access to a new project this would lead to:

* 100 rows being removed
* 101 new rows being inserted

This commit changes the way this system works so it only removes/inserts
what is necessary. Using the above example this would lead to only 1 new
row being inserted, with the initial 100 being left untouched.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/25257

Rename MWBS service to Merge When Pipeline Succeeds

## What does this MR do?

This MR renames Merge When Build Succeeds feature to Merge When Pipeline Succeeds, because we recently changed the behavior, see https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6675.

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG.md) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] All builds are passing

## What are the relevant issue numbers?

Closes #23354

See merge request !7135

By passing commit data to this worker we remove the need for querying
the Git repository for every job. This in turn reduces the time spent
processing each job.

The migration included migrates jobs from the old format to the new
format. For this to work properly it requires downtime as otherwise
workers may start producing errors until they're using a newer version
of the worker code.