Fix #1608: alphabetize videos in the download topic tree

Title says it all.  Admins don't need to care about the order of topics, they just need to be able to find the right topic to download.  So, alphabetize.

![screen shot 2014-02-22 at 9 42 28 pm](https://f.cloud.github.com/assets/4072455/2239747/b693068a-9c4e-11e3-8e25-a8447560e17c.png)