Add lessCSS filters for django-compress.

This introduces a lessCSS filter that can be used with django-compress.
It's compatible with our MEDIA_SERIAL versioning (which we may want to
just migrate over to proper django-compress versioning later).

Our lessCSS filter uses http://blesscss.cloudfoundry.com/min/ as the
default service for generating the CSS. It can be optionally used in
conjunction with other filters, though typically this one will need to
be first.

The one tricky part is imports. django-compress will combine all the
files in a given package into one blob of content to filter, with no
information on filenames or anything, making it difficult to handle
imports in a way that will work with both COMPRESS=True and False
(since, when False, the stylesheets are included directly).

What we do is make use of a settings.LESSCSS_IMPORT_PATHS that defines a
list of directories (relative to MEDIA_ROOT) to check for imported
files. We then process the file, looking for #import "foo" and try to
find that file to import. We can then just include that directly instead
of the #import statement.

We ship our own compress template files that know how to deal with
standard CSS and lessCSS stylesheets, and our MEDIA_SERIAL stuff.

It's currently up to the developer to provide MEDIA_SERIAL in a
COMPRESS_CSS package's "extra_context", and to include the lesscss.js
file when COMPRESS is False. I may end up making this a bit easier
later, but this gets us going for now.

Tested with my accompanying Review Board change in DEBUG and non-DEBUG
modes, with COMPRESS set to True and to False, and with manage.py synccompress.