Add "User" form element to payment method form

After #1549334: Add UID to payment methods is committed, we should allow users to select another owner of a payment method when editing it by adding a "User"/"Owner" form element to the payment method form.

We should also think about the consequences of changing the owner once a payment method has been created, because its configuration may contain confidential data. The same goes for users who edit other users' payment methods. At the very least the "edit any payment method" permission should be restricted.