Enable the js_compressor option for Sprockets

Because the default for Rails 6 is to use webpack for JS the Sprockets
js_compressor option is not set in the generated production.rb
environment file. This leads to bloated file sizes in production so
re-enable compression by explicitly setting it

config/environments/production.rb

@@ -22,6 +22,9 @@
   # Apache or NGINX already handles this.
   config.public_file_server.enabled = ENV["RAILS_SERVE_STATIC_FILES"].present?
 
+  # Compress JS using a preprocessor.
+  config.assets.js_compressor = :uglifier
+
   # Compress CSS using a preprocessor.
   # config.assets.css_compressor = :sass