Allow self-approvals in fallback approval rules

What does this MR do?

Allow authors of a MR to approve a MR if fallback approval rules are in play. Fallback approval rules are the rules used when there have been no explicit approvers set.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/7532

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• 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

Closes #7532 (closed)