Move reviewer modification to single, global location for merge request page
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[Part of Intelligent Reviewer Assignment]
Because each reviewer assignment dropdown makes a modification that fully replaces the existing list of reviewers, they must each necessarily maintain context of every selected reviewer, even if those reviewers are not eligible for the relevant approval rule nor can they be shown visually in the dropdown.
To facilitate better suggestions in each dropdown, the actual modification behavior should be moved out of the dropdown itself and to a higher-level location in merge requests. This way, the dropdown can focus on just showing the most relevant reviewer suggestions and other - mor general - logic can be responsible for merging the "main" list of reviewers with the more specific "rule" list of reviewers and submitting the changed list to the back end.