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.

config/locales/en/admin.yml

@@ -254,9 +254,13 @@ en:
         image: "Image"
         show_image: "Show image"
         documents: "Documents"
+      form:
+        add_language: Add language
+        remove_language: Remove language
       new:
         creating: Create milestone
         date: Date
+        description: Description
       edit:
         title: Edit milestone
       create: