Update icons to use latest set, remove icons that are no longer used

**Before:** PNG sprite size: 21,192kb, SVG images in /images/global: 120
**After:** PNG sprite size: 18,060kb, SVG images in /images/global: 85

This PR touches a few bits of the commercial components (search input, CTAs and buttons where I have consolidated various icons into one) and discussion (when I’ve updated the reply icon and removed one section of redundant SCSS `.d-actions__link`).

There are still quite a few more images that can be removed/consolidated, but this should be a good start.
This is what these links look like now: 
![screen shot 2014-08-22 at 11 02 07](https://cloud.githubusercontent.com/assets/813383/4009372/7c53bcb2-29e3-11e4-839a-3756d5668600.png)