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.

- 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

If no pipeline is created we currently have IID gap.
This is due to fact that we generate IID not on save,
but rather ahead of time. This results, us,
losing IIDs.

Properly expire all pipeline caches when pipeline is deleted

Closes #60469

See merge request gitlab-org/gitlab-ce!27334

When deleting a pipeline, only some of the cache structures were being
expired, but not the full pipeline list. We have to synchronously
schedule a pipeline cache expiration because the pipeline will be
deleted if the Sidekiq expiration job picks it up. To do this, properly
extract all the logic buried in the Sidekiq worker into a service, and
then call the service.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/60469

https://gitlab.com/gitlab-org/gitlab-ce/issues/60480

Previously the raw push option Array was sent to Pipeline::Chain::Skip.

This commit updates this class (and the chain of classes that pass the
push option parameters from the API internal `post_receive` endpoint to
that class) to treat push options as a Hash of options parsed by
GitLab::PushOptions.

The GitLab::PushOptions class takes options like this:

    -o ci.skip -o merge_request.create -o merge_request.target=branch

and turns them into a Hash like this:

    {
      ci: {
        skip: true
      },
      merge_request: {
        create: true,
        target: 'branch'
      }
    }

This now how Pipeline::Chain::Skip is determining if the `ci.skip` push
option was used.

`project.pipeline_status.has_status?` is cached, which can lead to Error
500s in the UI if the this condition is used to check whether a pipeline
exists for a commit. We now expire the cache to ensure that the
information is consistent.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/59453

Introduces the concept of Prerequisites for a CI build.
If a build has unmet prerequisites it will go through the
:preparing state before being made available to a runner.

There are no actual prerequisites yet, so current
behaviour is unchanged.