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/test.rb

@@ -31,7 +31,7 @@ Rails.application.configure do
   config.action_controller.allow_forgery_protection = false
 
   # Store uploaded files on the local file system in a temporary directory.
-  config.active_storage.service = :test
+  # config.active_storage.service = :test
 
   config.action_mailer.perform_caching = false