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 member, you can optionally unsubscribe.

From a UI perspective:

  - We don't show the subscriber list, since it's going to be some uninteresting subset of the member list.
  - We don't show CC transactions in history, since they're an uninteresting near-approximation of the membership transactions.
  - You only see the subscription controls if you're a member.

To do this, I've augmented `PhabricatorSubscribableInterface` with two new methods. It would be nice if we were on PHP 5.4+ and could just use traits for this, but we should get data about version usage before we think about this. For now, copy/paste the default implementations into every implementing class.

Then, I implemented the interface in `PhabricatorProject` but with alternate defaults.

Test Plan:
  - Used the normal interaction on existing objects.
  - This has no actual effect on projects, verified no subscription stuff mysteriously appeared.
  - Hit the new error case by fiddling with the UI.

Reviewers: btrahan

Reviewed By: btrahan

CC: chad, aran

Maniphest Tasks: T4379

Differential Revision: https://secure.phabricator.com/D8165