For smaller courses, there is a feature under the new provider
for grouping topics at the subsection level so that when
navigating the course, all topics under a subsection show up in
the sidebar instead of just the current unit.
This implements that functionality by checking if the discussion
is loaded in-context, and if grouping at subsection is enabled,
and if so, displaying the current subsection's topics.
Topics that belong to deleted sections/subsections/units in a course are considered archived. This commit adds a new UI section for such topics and lists them there.
