Optional configuration for marge to only process MRs that have all approvals

Enhancement request:

We've been using marge-bot for about 2 weeks and a couple of engineers have queried about some functionality. I had a look through existing issues and the code and can't see it implemented, but wonder if it could be an enhancement implemented as an optional configuration.

The details...

A couple of engineers came across this error when using marge-bot I couldn't merge this branch: Insufficient approvals (have: [] missing: 1), which is a valid error, I'm glad marge-bot doesn't merge without approvals 😀

However, we have a pretty common usecase in which we previously would use GitLab's Set to auto-merge to handle this (but of course, this doesn't handle the rebase like marge does!).

Here's the flow we'd love marge to handle:

1. Engineer has an MR and assigns to to another engineer/s for review
2. Engineer assigns marge-bot to the MR, knowing the MR is not approved yet, but the engineer would love the MR to be merged as soon as it IS approved!

I wondered if this logic might sit in merge_request.py fetch_all_mine.. Maybe there could be an alternate path where marge only fetches MRs that are both assigned to her AND all approval/s are obtained. This way marge only picks up and loops over MRs that have all the approvals then rebases/merges as needed. (I wouldn't worry about other cases just yet e.g. unresolved comments, just keeping it simple for only processing MRs that have all the approvals).

Cheers Tegan