Identify merge requests which are `Ready to be merged`

Problem to Solve

Some merge requests may be ready to merge even while reviews are still outstanding. This might be true if you assign 2-3 people to review a merge request, but only need one of those people to review and approve. Once that first person reviews and approves the merge request is ready to be merged, but that may not be clear since it could show that reviews are still requested.

Proposal

We should introduce a new section or status label that identifies merge requests which are Ready to be merged. These would be merge requests where all merge checks are satisfied and if a user with merge permissions were to visit the page they could click merge.