Switch to using jekyll-assets for asset compilation

I got bored of [seeing massive commits](https://github.com/alphagov/design-principles/commit/8584a50b4ba1fe5923159292f3baf778b05c4030) to update the manual, so I've tried to make the build idempotent(ish?) by using the `jekyll-assets` gem.

I know this is a pretty big thing that nobody was really asking for, so sorry about that.

This needs a bit of careful reviewing of the generated site.