Unconstrained Review Assignments

[ocielliottc]

Describe the bug
There is nothing stopping us from adding duplicate review assignments to the review_assignments table.

To Reproduce
Steps to reproduce the behavior:

1. Go to Reviews
2. Click on or add a Review Period
3. Change the members being reviewed multiple times, alternating between two members.
4. View the review_assignments table. There will be duplicate review assignments.

Expected behavior
There should only be one review assignment between reviewer and reviewee for a given review period.