Add explicit CSS ID on home page.

This is so that white-space: nowrap does not affect tables other than the home page.
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.