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Refs: #2363

remove is_accepted & date_accepted Invitation fields (substituted by consumed & date_consumed reps.)

add creditevent listener to set user ``has_credits`` flag and pass the information to ``authenticate`` response

Refs: #1824

- create im/migrations (for south use)
- fixture for django_site
- fix ExtendedUserCreationForm: use AstakosUser instead of django.contrib.auth.models.User
- do not redirect to profile page after login if request.user is superuser

- extend django user model
- pass template_name and extra_context in views
- incorporate context processors
- use different backends for invitations and simple workflow

Refs: #1823