Adding video counts / video available info

Currently, students can't find which topics have videos until drilling down all the way down.  This is fine if all videos have been downloaded, but extremely frustrating if not.

I've implemented two ways to get counts of videos: through the filesystem and through the database (hacky way, not through models).  Currently, the code uses the filesystem method for getting counts.

I've also implemented a VERY rough UI for this.  The hope is that someone else will step in and do a better UI, implemented properly (i.e. no inline style information)

The filesystem implementation should be efficient enough (I hope), but runs on EVERY drill-down through the topic tree.  I don't think caching at this level is necessary nor smart (since things can change under the covers), but glad to hear otherwise.