magit-rebase-popup: new popup

* Add a new popup for initiating and continuing a rebase,
  depending on whether a rebase is already in progress or not.
* Add new rebase-start command variants `magit-rebase-onto'
  and `magit-rebase-autosquash'.
* Add new rebase-continue command `magit-rebase-edit'.
* Change key bindings.
Merge branch 'n/rebase-popup-1' into next