Move all icon classes into separate elements

This fixes an issue on the editor form where the button styles overrode the icon font family resulting in the display text "5 Save". By having icons reside on separate elements we reduce the chances of styles bleeding into each other.

![screen shot 2014-07-22 at 10 52 36](https://cloud.githubusercontent.com/assets/47144/3655730/8fbc9144-117d-11e4-81ef-2d0da8d176a2.png)

@hypothesis/developers for review.