Add filtering merge requests by approvers

What does this MR do?

Adds filtering merge requests by individual approvers

Changelog entry added, if necessary
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Documentation reviewed by technical writer or follow-up review issue created
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Tested in all supported browsers
Conforms to the code review guidelines
Conforms to the merge request performance guidelines
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Conforms to the database guides
Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
EE specific content should be in the top level /ee folder
For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
Security reports checked/validated by reviewer

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