Queue after approval and successful try

Github introduced a "merge after all checks pass" feature that has been useful for me in repos with very long CI turnaround time. Likewise, it would be cool if mergit had a similar feature. I could see this as an extension to the following feature requests
- #142
- #146
- #199

Here are some thoughts about what that might look like:
* A new job type "try then merge" on the status page. It could be disabled if the PR is not sufficiently approved.
* After completion of the try job, add the PR to the merge queue.

Alternatively, my personal preference would be that it could react to approvals as well:
* "try then merge" button is always available.
* The botton puts the job in a state where the mergit will poll the statuses for approval and try job completon.
* Once both approval and try jobs pass, promote the PR to the merge queue.

The second approach sounds more appealing and matches the behavioir of github's feature

Start by reviewing the related requests #142, #146, and #199, then compare the proposed behavior with GitHub's merge-after-checks feature. Done means defining and implementing a resolved workflow for approval, try-job completion, and promotion to the merge queue, with acceptance criteria for the chosen approach.

Written by the indexing model from the issue text.