Consider how to handle cascading deletes

[brassy-endomorph]

Is your feature request related to a problem? Please describe.

We have database rows related via foreign keys. In the future they may also correspond to external objects (files, S3 objects). If a user deletes their account, we need to delete related files. Thus, making deletes cascade may leave objects behind.

Describe the solution you'd like

Four possible solutions:

1. Add ON DELETE CASCADE to the model so this logic is applied to the DB
2. Add cascade="all,delete,delete-orphan" to the ORM only so that deleting rows via Python (but not direct DML) cascades
3. Use 1 or 2 on rows that have no external objects and do manual deletes on rows with external objects
4. Always do it explicitly

I'm leaning toward 1+3 in that having cascade logic applied to FKs is senseible but that we want to have a worker queue model for when a user deletes an account with 5000 messages each its own external file in S3. This would support offline deletes in cases where a delete would time out an HTTP request.

Blocks (or at least strongly influences) #474