Apply reviewer changes from Herald via ModularTransactions

Summary:
Ref T10967. This converts the reviewer update action in Herald from an older edge write to a newer ModularTransactions write.

The major value from this is that we get a double-write to the new reviewers table.

Test Plan:

• Wrote a Herald rule to add a reviewer and a blocking reviewer.
• Saw them added properly to a revision with: no reviewers; both as blocking; A as blocking, B as nonblocking; A as nonblocking, B as blocking.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10967

Differential Revision: https://secure.phabricator.com/D17511