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

spec/serializers/current_user_serializer_spec.rb

@@ -50,7 +50,7 @@ RSpec.describe CurrentUserSerializer do
     end
 
     it "should include correct id in top_category_ids array" do
-      category = Category.first
+      _category = Category.first
       CategoryUser.create!(user_id: user.id,
                            category_id: category1.id,
                            notification_level: CategoryUser.notification_levels[:tracking])