Refactor Partisan::FollowHelper module to use a class instead

* Instead of having a `parent_class_name` instance method that we would inject in all *follower* and *followable* models, we now have a `Partisan::Helper` class with a single `parent_class_name` class method
* `Partisan::Helper.parent_class_name` now supports *presented* objects, used with gems Draper or Bourgeois (hat tip to @jdurand)