Remove milestone overrided translation class

Since Globalize gem update to v5.2.0 we cannot override translations
anymore in the same way that before the update. Milestone::Translation
class removed in this commit were no longer loaded correctly when
translation class is retrieved by translation_class method provided by
Globalize. Here is the diff between both gem versions:

https://github.com/globalize/globalize/compare/v5.0.0...v5.2.0#diff-a1370b109e0dd567545b072bc6447b8fR51

This problem is not happening on test environment but is throwing an
exception in other environments as it has not loaded the delegation
definition inside our custom translation class.

To fix this we added a new class method inside globalizable model
concern to allow to define method delegation on translations classes from
parent globalizable classes when needed without having to override
Translation classes.

Another way to properly load our custom Milestone::Translation class is
to place it inside parent model class, like the example below:

class Milestone
...

  class Translation
    delegate :status_id, to: :globalized_model
  end
end

Or maybe monkey patching translation_class method from globalize gem
to make it find our custom translation class. I don't like this option.

app/models/concerns/globalizable.rb

@@ -19,5 +19,9 @@ module Globalizable
       validates(method, options.merge(if: lambda { |resource| resource.translations.blank? }))
       translation_class.instance_eval { validates method, options }
     end
+
+    def translation_class_delegate(method)
+      translation_class.instance_eval { delegate method, to: :globalized_model }
+    end
   end
 end

app/models/milestone.rb

@@ -7,6 +7,7 @@ class Milestone < ActiveRecord::Base
 
   translates :title, :description, touch: true
   include Globalizable
+  translation_class_delegate :status_id
 
   belongs_to :milestoneable, polymorphic: true
   belongs_to :status

app/models/milestone/translation.rb

@@ -1,3 +0,0 @@
-class Milestone::Translation < Globalize::ActiveRecord::Translation
-  delegate :status_id, to: :globalized_model
-end