Fixing the bug

Make pipeline schedule worker resilient

Closes gitlab-com/gl-infra/production#805 and #61955

See merge request gitlab-org/gitlab-ce!28407

Disallow invalid MR branch name

See merge request gitlab/gitlabhq!3052

Currently, pipeline schedule worker is unstable because it's
sometimes killed by excessive memory consumption.
In order to improve the performance, we add the following fixes:

1. next_run_at is always real_next_run, which means the value
   always takes into account of worker's cron schedule
1. Remove exlusive lock. This is already covered by real_next_run
   change.
1. Use RunPipelineScheduleWorker for avoiding memory killer.
   Memory consumption is spread to the multiple sidekiq worker.

We've already migrated all the legacy artifacts to the new realm,
which is ci_job_artifacts table.
It's time to remove the old code base that is no longer used.

Currently, schedule head pipeline update method which executed after
pipeline creation does not take into account of merge reqeust
pipelines. We should use dedicated `all_merge_requests` method
in this case.

Continuation of 426488b7218e85ce69868ae4628801af2322b74a

- Adds an endpoint on PipelinesController
- Adds a service that iterates over every build in a stage and
plays it.
- Includes 'play_manual' details on EntitySerializer
- Builds a new Stage state: PlayManual. An stage can take this status if
it has manual builds or an skipped, scheduled or manual status
- Includes FE modifications and specs

Due to the nature of pipelines for merge requests, deployments.ref can
be a merge request ref instead of a branch name.

We support the environment auto-stop hook for this case