Use the new govuk_frontend_tookit grid helpers

These are a better version of the app specific grid
helpers used before, which can be removed entirely.

This basically replaces;
 - `@grid-container` with `%site-width-container`
 - `@grid-outdent` with `%outdent-to-full-width`

Overrides to the default styling provied by
`govuk_template` and `static` (via `slimmer`) have
been moved into their own scoped SCSS modules to
separate application SCSS from tweaks to external
SCSS.

The later should be easy to identify, should
external SCSS change and the override need updating.

It also documents where external SCSS defaults
aren't working for this app, and how the overrides
could be merged into the original SCSS.