block chat until user agrees to Community Guidelines

### Do not merge until @lemoness says to.

#### I would REALLY like someone other than me to test this.

This PR enables a feature that prevents a user from being able to send a message in the Tavern, party, or guilds, until the user has clicked a button agreeing to abide by the Community Guidelines.

Before the user clicks the "agree" button, the chat interface looks like this:
![screen shot 2014-11-01 at 11 10 48 am](https://cloud.githubusercontent.com/assets/1495809/4870358/04981de0-6164-11e4-8529-7190cf1006b4.png)

After they have clicked the "agree" button, it looks the same as it does on the live website now.