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.

spec/lib/cache_spec.rb

@@ -5,7 +5,7 @@ describe "Cache flow" do
   describe "Tag destroy" do
     it "invalidates Debate cache keys" do
       debate = create(:debate, tag_list: "Good, Bad")
-      tag = ActsAsTaggableOn::Tag.find_by(name: "Bad")
+      tag = Tag.find_by(name: "Bad")
 
       expect { tag.destroy }.to change { debate.reload.cache_key }
     end