Our friends at GitHub show the programming languages for a long time,
and inspired by that this commit means to create about the same
functionality.

Language detection is done through Linguist, as before, where the
difference is that we cache the result in the database. Also, Gitaly can
incrementaly scan a repository. This is done through a shell out, which
creates overhead of about 3s each run. For now this won't be improved.

Scans are triggered by pushed to the default branch, usually `master`.
However, one exception to this rule the charts page. If we're requesting
this expensive data anyway, we just cache it in the database.

Edge cases where there is no repository, or its empty are caught in the
Repository model. This makes use of Redis caching, which is probably
already loaded.

The added model is called RepositoryLanguage, which will make it harder
if/when GitLab supports multiple repositories per project. However, for
now I think this shouldn't be a concern. Also, Language could be
confused with the i18n languages and felt like the current name was
suiteable too.

Design of the Project#Show page is done with help from @dimitrieh. This
change is not visible to the end user unless detections are done.

This adds a recurring Sidekiq job that removes up to 50 000 old web hook
logs per hour, if they are older than 90 days. This will prevent the
web_hook_logs table from growing indefinitely.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/46120

The RepositoryCheck::DispatchWorker will start a
RepositoryCheck::BatchWorker for each healthy shard.

Closes gitlab-org/gitlab-ce#48042

We request Gitaly in a N+1 manner to build discussion diffs. Once the diffs are from different revisions, it's hard to make a single request to the service in order to build the whole response.
With this change we solve this problem and simplify a lot fetching this piece of info.

The NotificationService has to do quite a lot of work to calculate the
recipients for an email. Where possible, we should try to avoid doing this in an
HTTP request, because the mail are sent by Sidekiq anyway, so there's no need to
schedule those emails immediately.

This commit creates a generic Sidekiq worker that uses Global ID to serialise
and deserialise its arguments, then forwards them to the NotificationService.
The NotificationService gains an `#async` method, so you can replace:

    notification_service.new_issue(issue, current_user)

With:

    notification_service.async.new_issue(issue, current_user)

And have everything else work as normal, except that calculating the recipients
will be done by Sidekiq, which will then schedule further Sidekiq jobs to send
each email.

Also, refactor the mail sending slightly: instead of one worker sending all
emails, create a worker per project with issues due, which will send all emails
for that project.

Integrate two workers into one ArchiveTraceWorker with pipeline_background queue. This queue takes loqer precedence than pipeline_default.

EE: Trace as artifacts (FileStorage and ObjectStorage)

Closes #4171

See merge request gitlab-org/gitlab-ee!4258

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>