We now have `signup_advance_hours` in the configuration that
only allows signup for shifts so many hours in the future.
A notice is displayed in the shift view for shifts in the
far future, together with an info notice on when signup for
that shift will be possible.
In addition, we now have a new shift state `NOT_YET` for this
purpose so we do not have to abuse `SHIFT_ENDED` anymore for
`signup_requires_arrival` as well.
Thomas Rupprecht