Fix unsubscribing from projects in a gross, hacky way

Summary:
Fixes T5261.

This fix isn't very good. Two better fixes would be:

  # Add some sort of `setRole(SUBSCRIPTIONS)` method to `ObjectQuery`, which gets passed down until it reaches `ProjectQuery`, and `ProjectQuery` knows that it needs to load more data. This feels OK, but is a very general approach and I don't think we have many/any other use cases right now. I //think// this is the right way in the long run, but I'd like to have more use cases in mind before implementing it.
  # Add some sort of `loadAllTheSubscriptionStuffYouNeed()` method to `PhabricatorSubscribableInterface`. This feels OK-ish too, but kind of yuck, and doesn't lend itself to proper batching, and is silly if we do the above instead, which I think we probably will.

For now, just fix the issue without committing to an infrastructure direction. I think (1) is the right way to go eventually, but I want a better second use case before writing it, since I might be crazy.

Test Plan: Unsubscribed from a project.

Reviewers: joshuaspence

Reviewed By: joshuaspence

Subscribers: epriestley

Maniphest Tasks: T5261

Differential Revision: https://secure.phabricator.com/D9377