This fix the case where a worker is unsubscribed from a shift where he
is supercooperator.
In the function unsubscribed_from_today, when removing the supercoop,
the field is_regular was also set to False. I seams that this is not
correct as this field is related to the worker_id not to the supercoop.
Also, I found other issues as when removing a worker_id the fields
is_regular and is_compensation was not reset.
This let me think about other cases where worker_id is emptied, the
is_regular and is_compensation would be never been reset if the
programmer doesn't think to reset it.
So I modified the constrains on these field in order to have these field
reset properly when worker_id is changed.
Rémy Taymans
cf8fac7f37