post merge pipeline and environments status

What does this MR do?

EE port: gitlab-org/gitlab-ee!8104

This MR to expose post-merge pipeline and environments status information for the MR widget.

It implements what's described in https://gitlab.com/gitlab-org/gitlab-ce/issues/47799#note_108006884

Changes

1. GET :namespace/merge_requests/:id/ci_environments_status now supports an optional target parameters
   • when environment_target=merge_commit it returns the environments created by the merge commit
2. MergeRequest#merge_pipiline has been introduced, it returns the post merge Ci::Pipeline
3. GET :namespace/merge_requests/:id?serializer=widget now exposes also the merge_pipeline

Frontend

1. Renders post merge pipeline and deploys
2. Only polls when merge request is in the merged state
3. Updates deployment component to render different statuses.

Heads up in the screenshots: ignore the pipeline failed status, I was using mocked data.

	running	success	failed

What are the relevant issue numbers?

Closes #47799 (closed) #32736 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides