Add filter for merge requests by individual approver

What does this MR do?

Adds support for filtering merge requests by individual approver. I followed @cperessini 's design from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/620#note_26530945

Are there points in the code the reviewer needs to double check?

This is my first MR to GitLab, and also my first time with Ruby, ActiveRecord, HAML, etc. Please check everything 😜 .

Why was this MR needed?

To implement issue https://gitlab.com/gitlab-org/gitlab-ee/issues/1951 , a feature which my team and many others need.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conforms to the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• EE specific content should be in the top level /ee folder
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/1951