Switch to using django-pipeline and change our versioning.

This moves from django-compress, which is deprecated, to
django-pipeline. We only do pipeline work when we're on a production
system, or DEBUG is off. We fall back to using the less runtime on a
non-production DEBUG setup.

Rather than using MEDIA_SERIAL, we're now using Pipeline to version the
generated files. This will slightly reduce startup time, since we won't
have to stat the media files, and it also means we can version the files
individually instead of as a group. We'll have more fine-grained caching
as a result.

A future change will remove our use of MEDIA_SERIAL.

Tested with every mix of DEBUG and PRODUCTION. In each case, I inspected
the list of generated files and the source code to see what was being
used.

I also tested the resulting eggs when creating a new release (from a clean
Git tree). The resulting packages included the .less files, the compiled .css
(which we could remove, but it's probably not worth it), and the final
versioned files.