Rebase button
Description
How many times do you see in a merge request review that someone says "Sorry, can you rebase to master"? How about we provide a button in the web UI to do exactly that? We currently let people rebase and merge, but what about just rebasing without merging?
Note: I don't know if this is actually a good idea. It's kinda bad to rebase at all, and rebasing someone else's merge request is particularly dangerous as they might be actively developing and then risk losing their changes after the rebase.