Use some real templates in forum list view tests

Unfortunately, one template (thread-list-template, defined in
_thread_list_template.html) still relies on complex Mako processing, so
that is still being mocked in the tests.
Improve thread list view tests

An obsolete test case was passing because the assertions were too weak,
so the assertions are improved and the test case removed.
Ensure forum nav cohort selector works correctly

One minor case is fixed (to pass "" instead of "all" as the group_id
parameter to indicate viewing all threads), and tests are added.
Move more forum nav rendering logic to front end

The cohort selector is rendered only for a privileged user in a cohorted
course, and that logic is now in an Underscore template instead of Mako.