Add closed state

# Changes
 - Adds an 'enable voting' boolean setting, defaulted to true. When set to false, it hides vote buttons and voting form, and disables vote & user login routes.

Closes #255.
Yeah, that's what I originally had. It seemed confusing on the admin screen though to toggle something "on" in order to disable it...

![screen shot 2014-12-18 at 2 20 12 pm](https://cloud.githubusercontent.com/assets/583202/5494309/14500fd6-86c1-11e4-8dcc-156169464a8c.png)