Allow squashing MRs

Sean McGivernrequested to merge
squash into master

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.

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