Add GOVUK Frontend Sass, configured for this app

Like:

https://github.com/alphagov/govuk-frontend/blob/v2.13.0/package/all.scss

...but specifying the components to include,
starting with the ones that make up the
govuk_template and setting any variables needed.

app/assets/stylesheets/main.scss

@@ -31,6 +31,8 @@ $path: '/static/images/';
 @import 'elements/panels';
 @import 'elements/tables';
 
+// Dependencies from GOV.UK Frontend, packaged to be specific to this application
+@import './govuk-frontend/all';
 
 // Specific to this application
 @import 'grids';