Partitioning: Implement cascading deletes without foreign keys

With partitioning in PG11, we can't have foreign keys referencing a partitioned table (PG12 supports this). Since we'll minimally support PG11 for a while, we'll have to deal with this and implement other strategies for foreign keys.

This issue is about proposing and implementing a strategy to deal with cascading deletes without foreign keys (example: deleting from issues should clean up other tables referencing issues today).