DEV: Add support for Rails 6

Minor fixes to add Rails 6 support to Discourse, we now will boot
with RAILS_MASTER=1, all specs pass

Only one tiny deprecation left

Largest change was the way ActiveModel:Errors changed interface a
bit but there is a simple backwards compat way of working it

app/controllers/extra_locales_controller.rb

@@ -3,7 +3,11 @@
 class ExtraLocalesController < ApplicationController
 
   layout :false
-  skip_before_action :check_xhr, :preload_json, :redirect_to_login_if_required
+
+  skip_before_action :check_xhr,
+    :preload_json,
+    :redirect_to_login_if_required,
+    :verify_authenticity_token
 
   def show
     bundle = params[:bundle]