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>

When a project uses fast-forward merging strategy user has
to rebase MRs to target branch before it can be merged.
Now user can do rebase in UI by clicking 'Rebase' button
instead of doing rebase locally.

This feature was already present in EE, this is only backport
of the feature to CE. Couple of changes:
* removed rebase license check
* renamed migration (changed timestamp)

Closes #40301