Onboarding modal

This adds an onboarding modal message on the network front to any users with the hash fragment identifier ```#opt-in-message``` in their URL. (I.e. they have come from the R2 onboarding call-to-action).

This should not be merged until @cantlin has supplied his copy for it, but opening up PR for peer review.

### Desktop

![screenshot from 2014-01-29 13 46 19](https://f.cloud.github.com/assets/80089/2029799/d7e9f936-88eb-11e3-9e67-eb5de35d52aa.png)

### Mobile
![screenshot from 2014-01-29 13 46 54](https://f.cloud.github.com/assets/80089/2029800/ddb2854a-88eb-11e3-8475-04bac7267b30.png)

/cc @mattosborn @rich-nguyen