This class now uses a UNION (when needed) instead of plucking tens of
thousands of project IDs into memory. The tests have also been
re-written to ensure all different use cases are tested properly
(assuming I didn't forget any cases).

The finder has also been broken up into 3 different finder classes:

* ContributedProjectsFinder: class for getting the projects a user
  contributed to.
* PersonalProjectsFinder: class for getting the personal projects of a
  user.
* ProjectsFinder: class for getting generic projects visible to a given
  user.

Previously a lot of the logic of these finders was handled directly in
the users controller.