Move 'dynamic favicon' setting to User preference

Fixes [meta/7427](http://meta.discourse.org/t/favicon-notification/7427)

This moves the 'dynamic favicon' setting from a site setting to a user preference. It seems like we're still up in the air about whether to de-activate title notifications when the favicon notification is activated, so I left the current behavior (meaning no title notifications when favicon is enabled).

Current placement of the new preference is right above 'other'

![favicon](https://f.cloud.github.com/assets/65323/657807/d1196bf6-d58a-11e2-9731-408ebad5ddfe.png)