Add stylesheets to assets.excluded_paths

Otherwise, propshaft will include all stylesheets in public/assets, even though they are already compiled inside of application.css. This is a known bug:

rails/propshaft#89

config/initializers/assets.rb

@@ -7,3 +7,7 @@
 # application.js, application.css, and all non-JS/CSS in the app/assets
 # folder are already added.
 # Rails.application.config.assets.precompile += %w( admin.js admin.css )
+
+# Propshaft includes all stylesheets in public/assets when only application.css is needed
+# https://github.com/rails/propshaft/issues/89
+Rails.application.config.assets.excluded_paths << File.join(Rails.root, 'app/assets/stylesheets')