Allow runs to override the registration policy of events

[nbudin]

The idea of this is to let things like volunteer events, that have shifting levels of demand over the course of different runs, set different registration caps at different times during a convention.

Ideally, the volunteer event scheduling UI should allow some kind of automated definition for this - i.e. set a default and then say "for runs at these times, override to this level, for runs at these times, override to this level", etc.