implement activation of registered accounts

- caffeine.urls:
  - add CaffeineActivationView, drop ActivationCompleteView
- caffeine.views:
  - add remove unused ActivationCompleteView
  - add CaffeineActivationView that redirects to home page instead of a
    separate activation completion page
  - override RegistrationView.register in CaffeineRegistrationView to add the
    optional fields to new users
- coffeestats.urls:
  - remove registration_activate (moved to caffeine.urls)