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/views/budgets/investments/milestones/_globalize_locales.html.erb

@@ -5,10 +5,15 @@
                 class: "js-globalize-locale-link",
                 data: { locale: locale },
                 remote: true %>
+    <%= link_to t("admin.milestones.form.remove_language"), "#",
+                id: "delete-#{locale}",
+                style: show_delete?(locale),
+                class: 'float-right delete-language',
+                data: { locale: locale } %>
   </span>
 <% end %>
 
-<%= select_tag :translation_locale, 
+<%= select_tag :translation_locale,
                options_for_locale_select,
-               prompt: "Añadir idioma",
-               class: "js-globalize-locale" %>
+               prompt: t("admin.milestones.form.add_language"),
+               class: "js-globalize-locale" %>