An option to prevent merging a merge request when there is an unfinished review

Problem to solve

It is currently possible to 'merge' a merge request whilst there is an ongoing review. This can be problematic, as there is no visibility to the merging user that someone else has started a review.

Intended users

Unknown (all?)

User experience goal

There should be a setting on a repo that prevents (or warns about) merging an MR when there is an unfinished review on it.

Proposal

This could either take the form of a warning when clicking the merge button (e.g. "are you sure? There are 2 unfinished reviews on this merge request.") or flat out disabling the button and not allowing it until any reviews have been submitted.

It should probably be an optional setting that users can turn on and off to keep the current default functionality.