Drop foreign key constraint to Store table

Background

While implementing postgres table partitioning for the morango_store table, an overlooked restriction regarding primary keys on the store table popped up. Except under specific partition strategy circumstances, primary keys cannot be enforced on partitioned tables. For the morango_store table, this would be okay, except for any foreign key references to the PK field.

Since we expect the store PK field values to be unique regardless, we'd like to drop the sole FK constraint to the field.

Deliverables

• Update the store_model field on RecordMaxCounter to drop the FK constraint
  • The uniqueness constraint should remain and a rebuild of it should be avoided
• Bonus: it would be great to also add an explicit on_delete=models.CASCADE to all FK fields, since the existing implicit behavior is deprecated in Django 2+