Improve performance of user profiles, finding groups, and finding projects

This MR improves the following:

* Rendering of profile pages and Atom feeds
* Finding groups (using GroupsFinder & friends)
* Finding projects (using ProjectsFinder & friends)

Initially this MR was intended to only improve rendering of Atom feeds, but over time other fixes were introduced as well as the same code was the cause of all these problems.

See merge request !1790

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.