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
My biggest concern here is that we use an image for the tail of the speech bubble; I need to figure out the exact color I should be using and pull that out to a variable with an appropriate comment. Alternatively, we could try to find/make code for a CSS-based triangle and use an existing variable.