Allow squashing MRs

I still need to:

• Handle the case where the * branch is protected (ugh).
• Return early in the squash service when there is only one commit in the MR diff.
• Add this to the API.
• Add documentation.
• Add a changelog entry.
• Add some functional tests.
• Backport changes to CE:
  • Repository#merge method signature.
  • CSS changes for .merge-param-checkbox, removal of &.right a and addition of .modify-merge-commit-link.
  • Removal of right class from .accept-control which contains the "modify commit message" toggle.
  • Moving source branch deletion to its own partial.
  • Add early return and comment to that partial in EE.
  • MergeRequestsHelper#merge_params.
  • MergeService#log_merge_error provides the collapsed reference for an MR.
  • Add csv branch to test_env.rb.

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/150.