implement settings page

- caffeine.forms:
  - implement SettingsForm with special handling of password and email changes
- caffeine.models:
  - fix error in CaffeineUserManager.create_superuser
  - implement ActionManager with create_action and use it as Action.objects
- caffeine.urls:
  - add URLs for DeleteAccountView, ExportActivityView and ConfirmActionView
- caffeine.views:
  - implement dummy ExportActivityView
  - implement dummy DeleteAccountView
  - implement view logic in SettingsView
  - implement ConfirmActionView
- add EMAIL_CHANGE_ACTION_VALIDITY to coffeestats.settings.base
- add templates/js_sanitize_string
- add templates/registration/email_change_email.txt and
  templates/registration/email_change_email_subject.txt for email change emails
- add templates/settings.html