Configure builds

- Create `dartsass.builds` initializer and add all Sass files to be compiled. See https://github.com/rails/dartsass-rails#configuring-builds
- Create .keep
- Update manifest file to use /builds directory
- Ignore /builds

.gitignore

@@ -12,6 +12,8 @@
 !/log/.keep
 /tmp/*
 /coverage
+/app/assets/builds/*
+!/app/assets/builds/.keep
 
 node_modules
 yarn-error.log

app/assets/config/manifest.js

@@ -1,3 +1,3 @@
 //= link_tree ../images
 //= link_directory ../javascripts .js
-//= link_directory ../stylesheets .css
+//= link_tree ../builds

config/initializers/dartsass.rb

@@ -0,0 +1,8 @@
+APP_STYLESHEETS = {
+  "application.scss" => "application.css",
+}.freeze
+
+all_stylesheets = APP_STYLESHEETS.merge(GovukPublishingComponents::Config.all_stylesheets)
+Rails.application.config.dartsass.builds = all_stylesheets
+
+Rails.application.config.dartsass.build_options << " --quiet-deps"