This version component reflects the "version" of the StaticContent
objects which we cache server-side.  If the layout of those objects
changes between releases, errors occur when loading them from cache.

By using a separate version value, which can be incremented on its own
after a change has been made to the StaticContent class, we can avoid
loading older cached content and in turn take advantage of these changes
faster, without needing to intervene operationally.

When releasing the versioned assets work, we stumbled on a problem with old pickled
versions of the StaticContent objects residing in cache, which triggered a bug in the
code. Not wanting to blow away all cached items, we ended up having to revert and add
in some backwards-compatible helper code to ease the transition.

With this, we'll now utilize the version argument that Django's caching interface
allows, in conjunction with a constant value that can be modified when breaking changes
are being made, to let us gracefully ignore older cached course assets.