Add styles for destructive links

This colour is used in a few places so worth
making into its own block and classes.

Note: this establishes a
`app/assets/stylesheets/local` folder for any Sass
files whose names clash with files from vendored
libraries.

`_typography.scss` already exists in GOV.UK
Elements Sass.

This will no longer be needed when GOV.UK Elements
is gone and GOV.UK Frontend is at version 3, where
all its folders are in a `govuk` folder.

app/assets/stylesheets/main.scss

@@ -36,6 +36,7 @@ $path: '/static/images/';
 @import './govuk-frontend/all';
 
 // Specific to this application
+@import 'local/typography';
 @import 'grids';
 @import 'components/cookie-message';
 @import 'components/site-footer';