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