Ensure nav sidebar is sized correctly on page load
Improve focus management of forums nav sidebar

Now, when the list is reset (e.g. by sorting), the first thread is
focused. When the "Load more" link is clicked, upon loading more
threads, focus moves to the thread immediately following the thread
that was previously last in the list.
Merge pull request #1169 from edx/gprice/forum-load-more-focus

Improvements in forums nav sidebar