Cascading deletion in DB

[nihil2501]

Description

Even though I don't think most of the parent records described below should be getting deleted, if any did, there are child records that are nonsensical in the absence of their parent. We can add foreign key cascade delete for that and also a trigger in the polymorphic case. This is instead of using the :dependent option on ActiveRecord model associations or anything like that (can read about this in the Deleting from associations section of Rails API docs). Again, we're not going to delete the records and also buying into Rails's model-centrism for business logic and such needs a lot of motivation to actually opt in to in my opinion.

This deletion cascade should be defined in the DB:

• power_of_attorney_requests -> power_of_attorney_forms
• power_of_attorney_requests -> power_of_attorney_request_resolutions
• power_of_attorney_request_resolutions -> power_of_attorney_request_decisions
• power_of_attorney_request_resolutions -> power_of_attorney_request_expirations

Acceptance Criteria

• Model spec mostly for the dev to prove to themselves that their DB schema migration enacts this behavior correctly, but the spec will need to be in a follow up PR because of code checks that require migration PRs to contain no other kinds of changes