Resolve "Add draft check to automatic merge"

What does this MR do and why?

Adds the draft state to the list of states that we allow the merge request to be in when allowing merge when checks pass to be set.

We have removed the front end aspect for now while we decide on the best way forward, and introduced a new feature flag, so we can dogfoog the original merge when approvals work

Related to #420639 (closed)