Bug: Holding model should have ActiveRecord and DB null constraints

[zachgoll]

As seen in #1767, we're getting some invalid data in the DB because the Holding model does not validate amount.

We should ideally be adding both DB-level constraints and ActiveRecord validations on: date, qty, price, amount, and currency.

Since we have some invalid data states in the DB right now, this will require a migration to clear out invalid data prior to adding the null constraints at the DB level.