hide icons from screen readers 

### Story

* https://www.pivotaltracker.com/story/show/71501822
* https://www.pivotaltracker.com/story/show/71502040 (no longer an issue with icons hidden)

### Notes

* Font awesome icons [aren't going to be read by screen readers](https://github.com/FortAwesome/Font-Awesome/issues/389). So let screen readers skip over them using `aria-hidden="true"`
* Make sure each hidden icon has descriptive text available (either visible or `.sr-only`). I added a few bits of text in the last commit (4d1bf762eb5adc59ddc104e874d79e65ef0053da), need to make sure the information is correct

Don't have screenshots, but I can tell you the icons are no longer being read by OSX screen reader VoiceOver..