rails: disable asset pipeline.

Disable the assets pipeline (Sprockets). Sprockets errors out now after
upgrading to Sprockets 4 because of missing config files. We don't use
it any more after switching to Webpack, so we can disable it entirely.

Also disable a few more Rails features that we don't use (ActiveStorage,
ActionCable, ActionMailbox, ActionText).

config/environments/development.rb

@@ -29,7 +29,7 @@ Rails.application.configure do
   end
 
   # Store uploaded files on the local file system (see config/storage.yml for options).
-  config.active_storage.service = :local
+  # config.active_storage.service = :local
 
   # Don't care if the mailer can't send.
   config.action_mailer.raise_delivery_errors = false
@@ -48,10 +48,10 @@ Rails.application.configure do
   # Debug mode disables concatenation and preprocessing of assets.
   # This option may cause significant delays in view rendering with a large
   # number of complex assets.
-  config.assets.debug = true
+  # config.assets.debug = true
 
   # Suppress logger output for asset requests.
-  config.assets.quiet = true
+  # config.assets.quiet = true
 
   # Raises error for missing translations.
   # config.action_view.raise_on_missing_translations = true