Add feature to delete a translation

To delete a translation, a link has been added. This
link works for the selected language. It hides all the
things related to a language (the tab and the text_area)
and empties the text area, so that the value is blank
in the param hash. A variable called `delete_translations[]`
is changed.

e.g. If admin wants to remove English language,
delete_translations[:en] will be 1; if not, it will be 0.

When the milestone is updated, there is a before_action
callback that cleans the selected languages for deletion
(looking the delete_translations[] variable).
Because of the deleted translations are blank in param hash,
them won't be saved in DB.

app/controllers/concerns/translatable.rb

@@ -3,6 +3,7 @@ module Translatable
 
   included do
     before_action :set_translation_locale
+    before_action :delete_translations, only: [:update]
   end
 
   private
@@ -16,4 +17,15 @@ module Translatable
       Globalize.locale = I18n.locale
     end
 
-end
+    def delete_translations
+      locales = Budget::Investment::Milestone.globalize_locales.
+      select { |k, v| params[:delete_translations].include?(k.to_sym) && params[:delete_translations][k] == "1" }
+      milestone = Budget::Investment::Milestone.find(params[:id])
+      locales.each do |l|
+        Globalize.with_locale(l) do
+          milestone.translation.destroy
+        end
+      end
+    end
+
+end