This changes how we link merge requests to deployments so that:

1. We link merge requests for any deployment that is not in the
   "created" state.
2. We do not link merge requests to review app deployments.

We want to link deployments regardless of their status so that we can
also show failed and running deploys in the merge request widget (and
perhaps in other places). We don't want to link review app deployments
since this is not useful, and may lead to deployments to different
review app environments showing up on a merge request page.

This builds on the refactoring of the deployment model introduced in
merge request https://gitlab.com/gitlab-org/gitlab/merge_requests/20474,
and moves the linking of merge requests to Deployments::FinishedWorker.
This worker is scheduled every time a deployment transitions to a
finished state.

As part of these changes we also change Deployment#link_merge_requests
so that it ignores any already linked merge requests. This way if a
deployment changes its status multiple times, we don't error with
duplicate key errors.

This fixes https://gitlab.com/gitlab-org/gitlab/issues/38092.