Show targeted self-managed release in the MR widget

With #620 (closed) we are showing progression of any given MR for GitLab.com releases.

One of the last pieces of puzzle to ensure self-serving for anyone tracking any type of release is automating self-managed release tracking.

We currently announce via Slack announcement the last good commit we expect to ship. We should move this into deployment tracking.

Proposal

• Create release candidates versions at pre-defined schedule, and deploy automatically to pre.gitlab.com.
• Ensure that pre environment is tracked on each MR same way as #620 (closed)
• Allow creating or RC packages based on an arbitrary SHA or branch name, whichever is easier to implement

Challenges

• Current tooling expects the existence of a stable branch in order to create a release candidate.
• How often do we need to create release candidates to sufficiently inform people?
• Pre environment is currently used for patch releases, will we run into issues if we have a security release in progress?

/cc @gitlab-org/delivery