Remove django-compressor

And replace it with a gulp file that automatically rebuilds the css when the less files change and possibly more bells and whistles.

I feel that django-compressor adds too much runtime complexity for frontend developer convenience that she is now used to getting from gulp and friends. Also, gulp and friends do a much better job at being convenient for the modern frontend developer:

- sourcemap support
- freedom of choice to use or not use django-compressor in the frontend. Right now if you want to use django-compressor in the frontend, you have to use it in the dashboard as well.
- no confusion and complexity arising from the fact that django-compressor parses templates outside of their normal context, bypassing inheritance (Blocks `jquery-ui-(js|css)` by yours truly)

I'm implementing this as we speak.

Concerns #796.
Hmm, this was introduced with django-compressor, so I feel it's only logical that we remove it with django-compressor.

Also, I'm not sure what level of backwards-compatibility we should really strive for, given that 1) essentially all of oscar is public API due to its extensive customizability 2) our limited manpower. I will raise that point in the group.

In #1570 I plan to introduce a setting that allows to switch between local and CDN versions for scripts and CSS that are available on a CDN. This would make that block obsolete as well.

Thoughts?