Move pagination out of the model

  - The pagination logic is only needed in a single action in a single
    controller, so move it out of the model and into that one action.
  - Because pagination was the only customization that stratify-rails
    made to the Stratify::Activity model, we can remove that file from
    the Rails app altogether. (The core implementation of Stratify::Activity
    remains in the stratify-base gem.)
  - This also restores the desired pagination behavior (i.e., 200 per
    page, as opposed to the default of 25 per page) which got borked in
    commit 4d75b0e.

stratify-rails/app/controllers/activities_controller.rb

@@ -5,7 +5,7 @@ def index
     # TODO Simplify the logic below once this bug is fixed in Kaminari:
     #      https://github.com/amatsuda/kaminari/issues/145
     activity_criteria = Stratify::Activity.desc(:created_at)
-    @activities = Kaminari.paginate_array(activity_criteria).page(params[:page])
+    @activities = Kaminari.paginate_array(activity_criteria).page(params[:page]).per(200)
   end
 
   def destroy