Fix widget of locked merge requests not being presented

What does this MR do?

1. Fixes the bug found in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15018#note_44888376
2. Make MR merge_jid / unlocking less complex (by making it less stateful)

Why was this MR needed?

The MergeRequest locking issue still manifests in tricky ways when we try to:

1. Cleanup the MergeRequest#merge_jid
2. Unlock the merge request

The crux is that the merge_jid may be cleaned and the merge request remain locked. Therefore our unlocking worker can't find and update the state of the merge request, and as end result we may have the behavior in the screenshot below:

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?