This is what I have so far for the multi user collaboration on review requests. So far, what I have done is allow all reviewers (users in the target people and groups) to be allowed to change the review request. This would all the reviewers to all the normal changing review request functionalities. 

I'm not sure if we have decided who can be able to modify the review request. For now, I put them as all the reviewers.

Also, I've included the last_updated_by field to the review request model in order to display who has last modified the review request or published new review on the dashboard.

Manual testing. At first, I've ran the unit tests and had a bunch of errors and one failure. The errors are all expected due to the change of the ReviewRequestDraft.create() function and I need to look into that one failure. I think it's expected failure though.

As of diff 2, all the test failures/errors have been fixed.

Manual testing include: 
- Creating new review requests and changing submitter to another account. Permissions to edit the review request behaved correctly for new user
- The original author correctly still has permission.
- Changing the submitter back to the original submitter. Previous submitter has no permission to edit.
- Check Admin. Admin is always able to edit any review request.
- Discarding and undiscarding a review request. 
- Dealing with previous review request that existed before my change. The submitter field is populated correctly. Edit permissions acting properly before changing submitter. Changed submitter and still acting correctly.