Re-design forum thread list and add labels

There are currently two significant differences from the mockup:
1. The contents of each item are vertically aligned to the top instead of the middle.
2. The comments icon is always blue for unread comments (whereas in the mockup it is blue for an unread post or white with a blue border for a read post with unread comments).

@jimabramson @talbs @marcotuts 

FYI @aboudreault @antoviaque
I'd recommend rocking our font-size() mixin here (https://github.com/edx/edx-platform/blob/master/lms/static/sass/base/_mixins.scss#L5) - it'll currently output the same px value but when we're ready to turn on fluid type sizes across the LMS, the mixin will produce rem values as well.