Allow sign up window to be configured per-event

[itsrachelfish]

Currently there is a hardcoded restriction in the SlotController which prevents users from signing up for shifts if the event is going to start within 24 hours.

This project was originally designed for Apogaea which does not have internet access on site and they have to print out all of the volunteer information and bring it to the event. Volunteer shifts automatically close 24 hours before the event in order to prevent people from signing up after the volunteer shifts have been printed out.

Now that more events have started to use the volunteer database it would be helpful to allow this setting to be configured by admins when events are created.

[itsrachelfish]

Related to #89