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

lib/validators/url_validator.rb

@@ -15,7 +15,7 @@ class UrlValidator < ActiveModel::EachValidator
         end
 
       unless valid
-        record.errors[attribute] << (options[:message] || I18n.t('errors.messages.invalid'))
+        record.errors.add(attribute, options[:message] || I18n.t('errors.messages.invalid'))
       end
     end
   end