Rebase MR prior to approval.

• Zendesk ticket: https://gitlab.zendesk.com/agent/tickets/12951
• GitLab Version: 8.2.1

Expected Behavior

A MR that requires rebasing is first approved and then rebased.

Observed Behavior

A MR approval that leads to a rebase onto master changes the MR and requires a re-approval of the MR.

Next Steps

1. Rebase the merge request prior to approving it with a button from the MR page.
   Or
2. Use a setting that an approved MR would still be approved if a rebase occurred without any conflicts.

@JobV what do you think of this? /cc @vsizov