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.
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.
Merge pull request #4165 from edx/gprice/forum-nav-refine-bar-cleanup

Improve forum sorting and cohort filtering UX