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/new_post_result.rb

@@ -27,7 +27,7 @@ class NewPostResult
     if arr.empty?
       @success = true
     else
-      arr.each { |e| errors[:base] << e unless errors[:base].include?(e) }
+      arr.each { |e| errors.add(:base, e) unless errors[:base].include?(e) }
     end
   end