Filter available contacts by sub-type when assigning roles

In CiviCase, from the view page in the Roles section, using the "Add new role" allows the user to select a relationship, and then select a contact for that role. If the relationship is restricted to a sub-type, then the select2 box only shows contacts of that sub-type.

However, when assigning a contact to an existing role, the select2 is not filter according to the sub-type which allows contacts to be selected that are not of the right type and then the role/relationship create fails.

The enhancement is to apply the sub-type filter when assigning to existing roles.