Compress/combine our CSS and JavaScript files.

For too long, we've had many verbose CSS and JavaScript files on production
systems, which affected load time. We didn't have a good system in place
for minifying/combining files.

Now we use django-compress, which is a pretty awesome little module that
allows you to easily define packages of CSS/JavaScript files. When DEBUG
is on, the original files will be used in the templates, but when off,
the minified versions will be used.

django-compress will generally try to minify on demand if it doesn't
find the files, but we make sure to build these files as part of the
packaging operations. This ensures that our users will have the proper
minified versions.

The next step will be to add a new CSS filter class that processes
lessCSS files. I'll be adding this to Djblets as a utility filter for
django-compress, and then begin porting to less.

Tested each affected page in both compressed mode and debug mode. Everything
looked and acted correctly from what I can tell. This could stand some dog-fooding,
though, and some scripts (diffviewer.js, for example) always assumed it was in its
proper page before.