Add Waiting List model.

A user can potentially have one waiting list. That means that they're in
line, waiting for a ticket to become available. A waiting list has very
few attributes -- only the user id, the time joined, and whether or not
they're still waiting for tickets (`removed').
Fix indentation and whitespace on Ticket javascript.
Add id to ticket purchase form.

This disambiguates it from other forms that might appear on the same
page. (Namely, the waiting list form, which will come in a few commits.)
Add soft and hard limits for ticket purchases.

At the hard limit, the controller starts to reject ticket purchases. At
the soft limit, the view stops displaysing available tickets. This is
friendlier for the user, since it's unlikely they'll try purchasing
tickets, then have the model reject them later on.