`deleteUser()`: Add option to delete user-owned files

Currently, if a user owns any object in the storage, their account is not allowed to be deleted. The way to do it right now is by using triggers, cron jobs and such. This is not a trivial task, nor is it seamless.

Ideally, deleteUser() should accept an option akin to shouldSoftDelete, perhaps even named similarly, e.g. shouldHardDelete. From there on, there is a couple of options:

1. Start the deletion process asynchronously, then return the status.
2. Clear the owner column from storage.objects, marking those files for deletion at a later point, e.g. by a cron job.