R1:21de2b1a0c7c

Make Projects a PhabricatorSubscribableInterface, but with restricted defaults

Summary:
Ref T4379. I want project subscriptions to work like this (yell if this seems whacky, since it makes subscriptions mean somethign a little different for projects than they do for other objects):

- You can only subscribe to a project if you're a project member.
- When you're added as a member, you're added as a subscriber.
- When you're removed as a member, you're removed as a subscriber.
- While you're a…