Stop submissions and edits at a scheduled datetime

[carols10cents]

Whatever time we choose as a cutoff point, we should stop accepting submissions and edits-- refuse CollectController#creates/updates, redirect the edit links to go to show, and have a "sorry, we're closed!" action or something.

For short term fixes, we could have someone manually deploy a change or turn off the dynos or something at that time.

A neater, more sustainable fix would be to have a datetime setting on the event and make that change automatically.

[jnf]

There's already simple crud controls for Events hiding in the Control controller (ugh, I know). It'd be pretty trivial to either set an expiry date or just have a big off switch for the event.

[ninjapanzer]

I think adding an on/off switch and being able to optionally add expiration would be a wonderful thing. @gruntrebel and I are going to implement these over the weekend

[jnf]

Closed via #51.