REFACTOR LIST: Once we get cascading deletes from Prisma

[robin-macpherson]

Description

Currently, Prisma does not support cascading deletes and so we are having to write workarounds by manually deleting related data before deleting the thing. This issue is just to help us track the places where we end up having to do this so that we can easily go back through and clean up/refactor once we get native Prisma support for this.

cc: @Lanny

Places

• deleteThread mutation (comment.ts)
• deletePost mutation added in Enable post deletion #379