Add labels to forum thread list

A thread will be labeled if it is pinned, if the user is following it,
or if it was authored by a staff member or community TA. This also
slightly changes the color used for community TA labels.

Originally reviewed in #4072
Make "load more" in forums not appear incorrectly

The rendering of the link happens in an event handler, and the
collection's attributes were not getting set properly before the reset
event triggering the rendering occurred.

Originally reviewed in #4072
Improve forum sorting and cohort filtering UX

Thread entries now show an activity count that includes the original
post, and only one of the vote count and activity count is displayed
based on the user's selected sort. The sorting and filtering options now
both use a select and are somewhat more verbose, and the visual styling
of the sort/filter bar is updated.

Originally reviewed in #4165 with a bugfix in #4211
Update discussion tab sidebar header

The home button is removed, the search box is always displayed, and many
visual adjustments are made, along with a large amount of refactoring.

Originally reviewed in #4211