R1:7f99f2cde835

Add EditEngine + Modular Transactions for reviewers

Summary: Ref T11114. This one is a bit more complex, but I think I covered everything.

Test Plan:
- Added reviewers.
- Removed reviewers.
- Made reviewers blocking.
- Made reviewers nonblocking.
- Tried to make the author a reviewer.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11114

Differential Revision: https://secure.phabricator.com/D17050