Validate translations in banners

This change forces us to use nested attributes for translations, instead
of using the more convenient `:"title_#{locale}"` methods.

On the other hand, we can use Rails' native `_destroy` attribute to
remove existing translations, so we don't have to use our custom
`delete_translations`, which was a bit buggy since it didn't consider
failed updates.

app/controllers/concerns/translatable.rb

@@ -1,29 +1,13 @@
 module Translatable
   extend ActiveSupport::Concern
 
-  included do
-    before_action :delete_translations, only: [:update]
-  end
-
   private
 
     def translation_params(resource_model)
-      return [] unless params[:enabled_translations]
-
-      resource_model.translated_attribute_names.product(enabled_translations).map do |attr_name, loc|
-        resource_model.localized_attr_name_for(attr_name, loc)
-      end
-    end
-
-    def delete_translations
-      locales = resource.translated_locales
-                        .select { |l| params.dig(:enabled_translations, l) == "0" }
-
-      locales.each do |l|
-        Globalize.with_locale(l) do
-          resource.translation.destroy
-        end
-      end
+      {
+        translations_attributes: [:id, :_destroy, :locale] +
+                                 resource_model.translated_attribute_names
+      }
     end
 
     def enabled_translations