Queue after approval and successful try

[liam-ly]

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

• Have completed merge run start the next one #142
• Hide "Queue" action if PR is not approved #146
• Check if PR is mergeable before queuing #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