Implement "Auto Review" in packages with a "Subscribe" option
This change adds an "Auto Review" option to packages. Because adding reviewers/blocking reviewers is a little tricky, it doesn't actually have these options yet -- just a "subscribe" option. I'll do the reviewer work in the next update.
Created a revision in a package with "Auto Review: Subscribe to Changes". The package got subscribed.
Reviewed By: chad
Differential Revision: https://secure.phabricator.com/D15915