IDOC: Modifications to "update" process, including software update page

* **These are intermediate changes that break subtitles support and do not include all current video update changes - only appropriate for the IDOC-specific release branch.**
* **These changes do not work; we have migrated to another branch where things are working better.  However, this shows the general structure of our code, and 95%++ is in common with our later branch, which made small tweaks to unblock some issues here.**

This addresses #232, and is dependent on #556, #557, #517.  This is a dependency for #518; getting this in will allow that code review to happen.

Changes:
* Moved all update-related code (videos, subtitles) into the "updates" app/module
* Use the generalized "update" module to communicate updates in the "update" management command to a simple front-end webpage.
* Expose endpoints for downloading versions of KA Lite, and for communicating what versions are currently available.