Introduce setting to require requests for changes to be resolved to enable merge
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## Problem to Solve
Requesting changes is a strong signal that something needs to be resolved in a merge request prior to being merged. When a user requests changes, however, if that user is unable to re-review and then approve that the new changes a merge request can be stuck. In many cases this can be resolved by having a user with `Merge` permissions click `Bypass` on the merge request, but that's not always ideal.
## Proposal
We could introduce a setting similar to [`prevent merge unless all threads are resolved`](https://docs.gitlab.com/ee/user/project/merge_requests/#prevent-merge-unless-all-threads-are-resolved) which if toggled off wouldn't require a request for changes to be resolved.
- This setting should be defaulted `true` so that the default behavior is requests for changes do block merging
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