Switch to handling static assets with django-pipeline

Here the "static assets" in question are images, CSS and Javascript.
There are several motivations for this, including:

 - It allows us to drop the requirement to manually increment
   STATIC_GENERATION_NUMBER whenever you update static files, since in
   production files are fingerprinted with their hash.  This provides
   effective cache invalidation only when the files change.

 - It allows us to easily package up Javascript files that are always
   used together and minify them for serving in production mode - in
   development mode you still get the original unminified files.

 - When developing (when DEBUG is False) the SASS files will be compiled
   on-the-fly rather than having to compile manually or leave
   "compass watch" running.

In addition, this pull request moves the country-specific SCSS and CSS into
the corresponding country app.