Fix styling on sidebar View My Tags to match new Tumblr layout

Minor change, but it was annoying enough that I went and fixed it.

Before:
![screen shot 2014-11-15 at 22 40 42](https://cloud.githubusercontent.com/assets/1041456/5060252/cf3b825c-6d18-11e4-8361-6f54a28b5e21.png)

After:
![screen shot 2014-11-15 at 22 41 48](https://cloud.githubusercontent.com/assets/1041456/5060253/d8f6a59c-6d18-11e4-9708-e1377474949e.png)

Changed link :after pseudo-element to an actual icon element with nested :before pseudo-element to match how Tumblr's doing sidebar icons.

Added missing classes to fix CSS styling.

**Note:** Needs some cross-browser testing before merging, I don't have access to all the target browsers to check if this works everywhere.  I don't see why it wouldn't, but just in case.

Tested on: Latest Chrome (38.0.2125.122)