Fix overuse of `ON DELETE CASCADE`

[KavikaPalletenne]

We can't always CASCADE the deletes, because we want to keep things even if user accounts are deleted. If user is deleted, we put in a dummy id of -1 for example.

[KavikaPalletenne]

This also requires FE and all Postgres JOINs to handle the non-existing user case

[KavikaPalletenne]

Can be delayed until we add in user deletion functionality