This also brought along:
iterate_to_find (copied, see large comment explaining why)
activate_topic (extracted from a one-liner)
set_count (formerly stream_list.set_subject_count)
For get_topic_filter_li, we now pass in stream_li instead of
stream to decouple parent/child responsibilities between the
components.
Also, I made some s/subject/topic/ fixes.
This creates the new topic_list.js module, and the first
function that we extract is topic_list.update_count_in_dom().
This function needed to be decoupled from some non-topic-list
stuff which was overly complicated.
