Rebase merge requests borked in 8.0

Hey guys, grabbed the 8.0 release today, and saw that we still have something that resembles the old rebase merge request workflow. Unfortunately, when we attempt to start a rebase merge, it doesn't appear to go anywhere.

When a MR requires a rebase, I click the Rebase onto master, which then swaps with the message Rebase in progress... It can take a while. Reload at will. However a page reload simply displays the rebase button again. We've attempted the process several times, with varying amounts of time between refreshes, and the results are the same.

Per previous discussions on the feature, we can confirm that this is not caused by a largely, unwieldy repo: our test case is 15 MB in size, with 1120 commits.