Add support for sub-events on different dates

[aviupadhyayula]

Certain events will have multiple "showings" (such as a performance that happens on both Friday and Saturday). When checking out, buyers should be able to choose which date they're purchasing a ticket for.

To do this, we'll have to change how events work on the backend. (A first pass might be to make a parent class representing events, and then a one-to-many relationship from an event to its distinct showings.)