You only want to compile Less files if you are developing the CSS of
Oscar.  Most people don't do this and so we default to serving
compressed CSS without Less compilation.  This also means that
the many contributors don't need to bother installing node.js and lessc,
which makes the installation process much simpler.

This change also re-introduces the `USE_LESS` setting which controls
whether Less files are used or not.

This involves some changes to the template block structure to make it
easy to compress the correct files.

* A new block cdn_scripts has been introduced as CDN-loaded scripts
  can't be compressed.
* A new setting 'USE_LESS' controls whether to use the LESS-generated
  files or the compressed files (which will need to be generated
  manually as they aren't in source control).
* A fallback for jQuery UI is added
* The CSS block was renamed to be clearer
* The bootstrap CSS was brought out of the LESS file into the page.
  This makes it easier to compress.

To make it trivial to change the logo and branding without forking
templates.