Use static assets from correct environment

Currently we use production CDN assets in every environment. This change
will let us use preview assets in preview, etc.

Adds a new environment variable CDN_URL. Still defaults to production in
lib/filters/__init__.py.
Update govuk_template

Use the latest version of GOV.UK template, which includes a slightly
different looking header.

This change also brings out some of our includes into separate files
to make future updates of govuk_template easier.