No way to move out of devmode without updates or reinstall

[Alberto Donato]

When a snap is installed with --devmode, there doesn't seem to be a way to refresh/switch it out of devmode, unless there's an actual update to perform (e.g. via refresh).

For the maas snap we've transitioned from a --devmode only snap to a properly confined snap, but users currently on devmode won't get updates until they explicitly move out of it, which they can't do unless there's a more recent revision available.