Re-design forum thread list

Use larger and lighter title font, decrease padding, and use lighter
solid backgrounds instead of gradients. Also, refactor SASS code to
conform better to our new style, including using more specific class
names.
Add labels to forum thread list

A thread will be labeled if it is pinned, if the user is following it,
or if it was authored by a staff member or community TA. This also
slightly changes the color used for community TA labels.