Add initial front end handling of question threads

@andy-armstrong, please review
As far as I could tell this is being used in 1 place in this file, and this color is specific enough to a given UI, state, and context that it is unlikely to appear more than twice.  We do occasionally have colors that it makes sense to use this intermediary class for (eg: $forum-staff-color), but in those cases we expect it to be used in a number of different places or contexts. 

For that reason this should change to be just our $green.