Atc sidebar folder styling

Adds sidebar styling when a folder is selected instead of a book.

# Testing
- [x] open a book and verify sidebar styling still shows up ("Table of Contents", minimize/maximize, background color)
- [x] Run the following in a console: `$('#sidebar > div[data-media-type]').attr('data-media-type', 'application/vnd.org.cnx.folder')`
- [x] verify the text is now "Folder Contents" and gray
- [x] minimize and expand the sidebar and verify the text remains "Folder Contents"
- [x] select a different book and verify the original color/text ("Table of Contents") returns