Apply Owners reviewers using ModularTransactions
Summary: Ref T10967. See that task for some discussion. This lets us do double writes on this pathway.
Test Plan: Set an Owners package to auto-review. Created revisions which triggered it: one with no reviewers (autoreview added); one with the package as a blocking reviewer explicitly (no automatic stuff happened, as expected).
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T10967
Differential Revision: https://secure.phabricator.com/D17512