Offer to undo room leave and forget actions
Currently the Leave Room
action in the room context menu, and in the sidebar with !946 (merged) don't ask a confirmation or propose to undo.
According to the GNOME HIG, the preferred flow is to show a toast with an undo action.
To do that, since forgetting is final in the matrix spec, and it's not always possible to rejoin a room that was left, the idea would be to show the requested state in the interface, but actually do the requests only when the toast is dismissed.
For that we need to either do #857 (closed) or make our implementation have a similar API to AdwToast.