Finalising the user signup experience

* Updated the routes with profile finish_signup view
* Added controller logic to users controller for finish_signup
* Added view markup for users/finish_signup.html.slim
* Added ensure_signup_complete method to app*_controller.rb to
  be used as before_filter to ensure user has provided all the
  necessary information before accessing a specific resource:
  e.g. before_filter :ensure_signup_complete, \
                     only: [:new, :create, :update, :destroy]