proposed initial support for site-wide Difficulty settings

Goes along with https://github.com/HabitRPG/habitrpg-shared/pull/360

Note that this is going into the "difficulty" feature branch, so it won't go live any time soon.

Trello card: [SITE DIFFICULTY SETTINGS v1 on HabitRPG | Trello](https://trello.com/c/C7CXSo02/428-site-difficulty-settings-v1)

![screen shot 2014-11-15 at 12 27 34 pm](https://cloud.githubusercontent.com/assets/1495809/5056225/4e094f84-6cc3-11e4-932a-adc11456da5f.png)