Allow rebase when MR is from a fork

• Zendesk ticket: https://gitlab.zendesk.com/agent/tickets/12424
• GitLab version: 8.2

Expected behavior

If FF merge and rebase options are enabled and a user submits a merge request from a fork, users from the main repository should be able to click rebase.

Observed behavior

Users in the main repository see that a rebase is necessary but there is no button. Similarly, the author of the merge request cannot see the button either. Recently we changed the way merge requests work and refs are now copied when the MR comes from a fork, right? In this case, can we allow the main repo members to rebase the copied refs? If not, can we display the button for the fork members?

Next steps

cc/ @vsizov What do you think?