Use `cached_db` session engine to persist sessions
Created by: xitij2000
Switch to using django.contrib.sessions.backends.cached_db
for sessions so redeploying an Instance doesn't lead to expired sessions.
Merge deadline: ASAP
Testing instructions:
- Deploy an instance using an Ocim with this path
- Sessions running on that AppServer for that instance should persist as a new AppServer is launched, or if the running AppServer is restarted.