Add option for zero-size quotas

[PurkkaKoodari]

In some events we wish to have reserved slots for some quotas, but put some people directly in the open quota.

For Ilmomasiina's design, the simplest way to implement this is a zero-size quota. This will require allowing and handling NULL sizes for quotas separately from zero.

In the UI, we should hide zero-size quotas' progress bars and signup lists, but show signup buttons (as there is no button for the open quota).