Use Uglifier with harmony in order to allow ES6 syntax. (wyeworks#2227)

# Release Notes

Tech task: Allow ES6 syntax in JS.

# Additional Context

This error came up on open stage: `Uglifier::Error: Unexpected token: keyword (const). To use ES6 syntax, harmony mode must be enabled with Uglifier.new(:harmony => true)`

config/environments/production.rb

@@ -25,7 +25,7 @@
   config.public_file_server.enabled = ENV["RAILS_SERVE_STATIC_FILES"].present?
 
   # Compress JavaScripts and CSS.
-  config.assets.js_compressor = :uglifier
+  config.assets.js_compressor = = Uglifier.new(harmony: true)
   # config.assets.css_compressor = :sass
 
   # Do not fallback to assets pipeline if a precompiled asset is missed.