Add SASS compilation, replacing static CSS

- Compiled by ./build.sh, but this might be a bit heavy weight (re-installing all node_modules with each CSS build
- Pulls in FET for common helpers
- Basic grid/container layout, enforce max line length
- No styling of page content yet
Apply specific styling required for this service

Doesn't aim to be 100% like the origin, but to have
the same behaviour and rough visual design. As a
distinct service it shouldn't 100% like GOV.UK or
rely havily on it's patterns.

Where possible i've used things from FET and elements
rather than taking from GOV.UK itself.

Roughly replicates GOV.UK styling for:

- Base font size
- Button styling (including transaction style button)
- Addresses
- Info callout
- contact callout (used on drop off)
Document build script use and manual testing

Explains how the build script is used, for setting up the initial application, and
that changes to node_modules and SCSS files will need to be rebuilt and the output
commited.

That isn't ideal, and those files should be treated as generated, built during
setup and/or deploy, rather than commited to the repo itself. Fixing that is
outside the scope of this change, so i'm going to great an issue on the repo
to track that suggestion.

This also adds instructions on manually testin the done/ pages locally, without
having to go through the payment provider. This was extrmely useful while
fixing the application CSS, and I suspect will be useful to someone else in
the future.
Merge pull request #31 from alphagov/fix-application-css

Make application CSS maintainable