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
is there any way to override this without using ```!important```? 

* If so, I'd say let's roll with that.
* If not, it may be time to make a /lms/static/sass/discussion/_shame.scss file and place all overrides (especially those with ```!important``` there for clean up/awareness)