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/production.rb

@@ -26,7 +26,7 @@ Rails.application.configure do
   # config.assets.css_compressor = :sass
 
   # Do not fallback to assets pipeline if a precompiled asset is missed.
-  config.assets.compile = false
+  # config.assets.compile = false
 
   # Enable serving of images, stylesheets, and JavaScripts from an asset server.
   # config.action_controller.asset_host = 'http://assets.example.com'
@@ -36,7 +36,7 @@ Rails.application.configure do
   # config.action_dispatch.x_sendfile_header = 'X-Accel-Redirect' # for NGINX
 
   # Store uploaded files on the local file system (see config/storage.yml for options).
-  config.active_storage.service = :local
+  # config.active_storage.service = :local
 
   # Mount Action Cable outside main process or domain.
   # config.action_cable.mount_path = nil