* Prevented stale session files from being loaded
* Added removal of stale session files in django-admin.py clearsessions
Thanks ej for the report, crodjer and Elvard for their inputs.
Backport of 5fec97b from master.
This removes a dependency of 'core' on 'contrib'.
Backport of 83ba0a9 from master.
This deprecation occurs after the alpha, but it's a prerequisite
for fixing decently #18194 which is a release blocker.