[Feature] Write a scheduled job to clean up unreferenced lois and submissions.

We made updating survey details & adding jobs & deleting jobs in one transaction in #1396. However, firestore has the [limitation](https://firebase.google.com/docs/firestore/manage-data/transactions#security_rules_limits) of accessing 20 docs in one transaction. So we had to call the deletion of associated LOIs and submissions asynchronously leaving chances of having unreferenced LOIs and submissions.

This ticket is about writing a scheduled job to clean up these unreferenced lois and submissions.