Currently when GETting comments, the thread of the comment will be
marked as read. This change makes this effect optional as well as
setting the default to not mark the thread as read.
Discussion API for thread sorting now takes the sort_by
parameter for the sort key such as by comment_count or
vote_count. Also takes the sort_direction parameter for
sorting the thread results.
This is an initial implementation that only allows retrieval of all
threads for a course and only returns an easily computed subset of the
fields that are needed, in order to keep this change from getting too
large.
JIRA: MA-641