policies: rewrite cache_key to prevent wrong cache

# Conflicts:
#	passbook/core/signals.py
#	passbook/policies/engine.py
#	passbook/policies/process.py

passbook/policies/apps.py

@@ -1,4 +1,6 @@
 """passbook policies app config"""
+from importlib import import_module
+
 from django.apps import AppConfig
 
 
@@ -8,3 +10,7 @@ class PassbookPoliciesConfig(AppConfig):
     name = "passbook.policies"
     label = "passbook_policies"
     verbose_name = "passbook Policies"
+
+    def ready(self):
+        """Load source_types from config file"""
+        import_module("passbook.policies.signals")