Implement "Auto Review" in packages with a "Subscribe" option

Summary:
Ref T10939. Ref T8887. This moves toward letting packages automatically become reviewers or blocking reviewers of owned code.

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.

Test Plan:
Created a revision in a package with "Auto Review: Subscribe to Changes". The package got subscribed.

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Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T8887, T10939

Differential Revision: https://secure.phabricator.com/D15915