Add Herald support for projects

Summary:
Ref T10054. Ref T6113. I'm going to remove subscribers from projects to fix the confusion between "watch" and "subscribe".

Users who have unusual use cases where they legitimately want to know when a project's description is updated or members change can use Herald to follow it.

This is also useful in general and improves consistency, although I don't have too many use cases for it.

Test Plan: Wrote a Herald rule, edited a project, saw the rule fire and send me email about the change.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T6113, T10054

Differential Revision: https://secure.phabricator.com/D15061