Revert "*: providers and sources -> channels, PolicyModel to PolicyBindingModel that uses custom M2M through"

This reverts commit 7ed3ceb960.

passbook/policies/engine.py

@@ -7,8 +7,7 @@ from django.core.cache import cache
 from django.http import HttpRequest
 from structlog import get_logger
 
-from passbook.core.models import User
-from passbook.policies.models import Policy
+from passbook.core.models import Policy, User
 from passbook.policies.process import PolicyProcess, cache_key
 from passbook.policies.types import PolicyRequest, PolicyResult