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