Merge Request on forked projects

The good:

 - You can do a merge request for a forked commit and it will merge properly (i.e. it does work).
 - Push events take into account merge requests on forked projects
 - Tests around merge_actions now present, spinach, and other rspec tests

The questionable:

 - Events only know about target projects
 - Project's merge requests only hold on to MR's where they are the target
 - All operations performed in the satellite

The bad:

  - Satellites always always recreate
  - Duplication between project's repositories and satellites (e.g. commits_between)

(for reference: http://feedback.gitlab.com/forums/176466-general/suggestions/3456722-merge-requests-between-projects-repos)

Change-Id: I5502883b8b57df7fe16dc7253db20e5ca7b7f702