Use Tag instead of ActsAsTaggableOn::Tag

It's shorter, it's easier to extend its behaviour, and it's easier to
integrate with other parts of our application, like translations.

app/controllers/budgets/ballot/lines_controller.rb

@@ -64,7 +64,7 @@ module Budgets
         end
 
         def load_categories
-          @categories = ActsAsTaggableOn::Tag.category.order(:name)
+          @categories = Tag.category.order(:name)
         end
 
         def load_ballot_referer

app/controllers/budgets/investments_controller.rb

@@ -146,7 +146,7 @@ module Budgets
       end
 
       def load_categories
-        @categories = ActsAsTaggableOn::Tag.category.order(:name)
+        @categories = Tag.category.order(:name)
       end
 
       def load_content_blocks