Allow MR authors to approve their MRs

What does this MR do?

• Allow MR authors to approve their own MRs if the project settings is enabled for it.
• Stop reducing the number of approvals needed according to project member numbers (which removes expensive queries).

Reviewer should double-check

By making #4738 (closed) we're also removing the ability to automatically reduce the number of approvals needed when the project has not enough members to approve the MR. That is, users should now manually adjust approvals numbers on project and/or merge requests.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• 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?

Closes #4738 (closed) and #3349 (closed)