refactors stats and optimizes js

JS libraries for stats (d3, c3, etc.) only load now when needed (stats/show)

app/controllers/stats_controller.rb

@@ -1,14 +0,0 @@
-class StatsController < ApplicationController
-  before_action :verify_administrator
-  skip_authorization_check
-
-  def show
-    @event_types = Ahoy::Event.group(:name).count
-  end
-
-  private
-
-    def verify_administrator
-      raise CanCan::AccessDenied unless current_user.try(:administrator?)
-    end
-end