Do not check repos without push events
Reduce false positives from automatic repository checks by skipping
projects without push events.
See merge request !4684
Don't fail builds for projects that are deleted when they are stuck
## What does this MR do?
Solves when dropping stuck connection.
```
Dropping stuck pending build 1545510 for runner
NoMethodError: undefined method `origin_merge_requests' for nil:NilClass
from /opt/gitlab/embedded/service/gitlab-rails/app/services/merge_requests/base_service.rb:50:in `merge_request_from'
from /opt/gitlab/embedded/service/gitlab-rails/app/services/merge_requests/base_service.rb:57:in `each_merge_request'
from /opt/gitlab/embedded/service/gitlab-rails/app/services/merge_requests/add_todo_when_build_fails_service.rb:5:in `execute'
from /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/metrics/instrumentation.rb:164:in `execute'
from /opt/gitlab/embedded/service/gitlab-rails/app/models/commit_status.rb:51:in `block (2 levels) in <class:CommitStatus>'
from /opt/gitlab/embedded/service/gem/ruby/2.1.0/gems/state_machines-0.4.0/lib/state_machines/eval_helpers.rb:79:in `call'
...
```
This happens, because that `default_scope` of `Projects` filters returns the projects that are not deleted, where `Ci::Build` doesn't take into account.
See merge request !4609
EmailsOnPushWorker was using the default queue, which made it impossible
to prioritize MergeWorker and other key workers with a separate Sidekiq
process.
The repository push email can be very expensive to generate, especially
with syntax-highlighted diffs. Instead of generating the email for each
recipient, generate one email object and reset the Message-Id and To
headers for each recipient. (Cloning would also be expensive in the case
of large emails, although probably not as bad as generating from
scratch.)
Added System Hooks for push and tag_push
Implements `push` and `tag_push` events for System Hooks.
They are based on Webhooks Events but without any deprecated item. We don't send commits too.
We are implementing this, to be used by Geo repository syncing (gitlab-org/gitlab-ee#76).
These hook events will be sent only when the respective configuration flags are set to true.
UI changes to admin screen will be made in another MR.
See merge request !3744
Douwe Maan
Alejandro Rodríguez
Robert Speicher
James Lopez
Jacob Vosmaer
Stan Hu
Kamil TrzcinskiandPhil Hughes
James LopezandRobert Speicher
Douwe Maan
Rubén Dávila
Sean McGivern
Jacob Vosmaer
Dmitriy Zaporozhets
Gabriel MazettoandDmitriy Zaporozhets