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.
Use compass installed from a gem
Fix tests after the switch to django-pipeline
Allow compass to be used via wsgi with DEBUG = True

If DEBUG is True, then django-pipeline will use compass to compile .scss
files on the fly, but for this to work we need compass to be on the PATH
and for GEM_HOME to correctly set in the environment.  This commit adds
a config option GEMS_DIRECTORY (which correponds to GEM_HOME) - if that
is set, then wsgi.py will set up those environment variables.
Use yui-compressor (available in Debian / Ubuntu) instead of yuglify

The default Javascript / CSS compressor in django-pipeline is yuglify,
but that has to be installed via npm, which would drag in node as a
dependency - instead use the older yui-compressor which is available as
a Debian package.
There should be a trailing newline in the file
Remove the generated CSS, now being dealt with by pipeline
Move country-specific scss into each country's app

This is much clearer than keeping all the country-specific SASS and CSS
in the core application's static directory.
Refer to core SCSS imports with relative paths
Fix whitespace: remove a trailing empty line in a file
Use django-pipeline compressed and fingerprinted Javascript
Remove an unused {% load staticfiles %}
References to STATIC_GENERATION_NUMBER are no longer needed
Switch uses of STATIC_URL to the static template tag

This is so that the fingerprinted URLs will be used in production.
Ignore generated CSS files in the repository tree
Make sure a more recent compass is present by installing as a gem

This is based on sayit.mysociety.org's post_deploy_actions.bash

We also explicitly install compass's dependencies, since for some reason
I can't fathom at the moment, they aren't being installed automatically.
Strip some trailing whitespace
Make sure there are no newlines before the doctype

While newlines are allowed before the doctype, [1] this will make
older versions of IE switch to quirks mode, so it's best to avoid it.

[1] http://www.w3.org/TR/html-markup/documents.html#conformant-documents
Ensure that pombola_run_when_document_ready is defined before use

The extra_js URLs were being loaded before
pombola_run_when_document_ready was defined - this meant that a file
like map.js, which tries to call pombola_run_when_document_ready would
error.  This commit makes sure that it's defined before any of the other
Javascript runs.