Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

These changes were added in GitLab EE commit
d39de0ea. The tests were a bit bugged
(they used a non existing group, thus not testing a crucial part) which
I only noticed when porting CE changes to EE.

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.

Encapsulates the logic for `Gitlab::Access::WHATEVER` levels.

Signed-off-by: NJeroen van Baarsen <jeroenvanbaarsen@gmail.com>

Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Main purpose is move big amount of methods from user, group, project
models and place filtering logic in one place.
It also fixes 500 error on group page for PostgreSQL
Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>