Automatically change to selection-strategy "manual" after sign-off window is closed

[lujobi]

If someone manually deletes a signup of a fcfs event the first place of the waiting list is added to the waiting list. In almost all short term cases the the affected person can't attend. This is often realized to late, as the affected person needs to answer to the mail first.

Thus I would suggest to change "fcfs" to "manual" automatically after the sign-off window has been closed.

[NotSpecial]

Concretely, I recall the proposal to suspend queue updates after the signoff date. This made sense to me, as after the signoff date, the queue is only managed by the admins anyways.

I think the easiest solution would be to add an additional check here: https://github.com/amiv-eth/amivapi/blob/master/amivapi/events/queue.py#L31

This is, check not only that the event is fcfs, but also that the signoff window is not yet over.

[NotSpecial]

Actually, do we even have an explicit signoff date? I think I might have mixed up a few things here. When is the last time people can delete their signup?

[temparus]

The signoff time is coupled with the registration window. It is not possible to sign off after the register_end time.

I'm not sure if this is also checked with the signoff link which was part of the info mail the users received after signup. This should be checked.