Server-side rebase for topics
Merge requests with the "fast-forward" and "semi-linear" methods display a "rebase" button if the history is not linear, but it isn't working because Mercurial rebase has not been implemented in Heptapod. I was vaguely remembering we hid the button, but it's not the case on Heptapod ≥ 0.12 (hence the "Bug" label).
There is also a Merge Request rebase API method.
To avoid some complications, we can for now consider only the case of topics – with the default settings they are logically equivalent to drafts.