Update sidebar nav styling

Fixes #251 and #548 

We only have two sidebars relevant for nav styling issues, About and Feature Pages, and they are very different (Feature Pages sidebar is more of a table of contents with nested links), so the styling is more customized for the Feature Pages.

I added a mixins SCSS file because I wanted to create and use a mixin for common hover link styling and didn't see that we had any existing custom mixins elsewhere.

![resources___maps_of_africa__a_virtual_collection_-_blacklight](https://f.cloud.github.com/assets/101482/2467769/06a0e8e2-afc6-11e3-965f-b8494e136e34.png)

![caroline_batchelor_collection___maps_of_africa__a_virtual_collection_-_blacklight](https://f.cloud.github.com/assets/101482/2467773/0ac260cc-afc6-11e3-992d-4dbd1a0cda64.png)

![batchelor_collection_sub-section_b___maps_of_africa__a_virtual_collection_-_blacklight](https://f.cloud.github.com/assets/101482/2467774/0d094062-afc6-11e3-9f22-700e7cad8d2a.png)