CRM-14995 contributions disappearing on pledge edit

This PR tightens the definition of whether a pledge is editable. Previous logic said "if there are only 2 pledge payment statuses associated with a pledge & 1 is Pending then it is editable (implicitly the other status must be Overdue). This PR makes the 2 acceptable statuses explicit & any other statues will result in the pledge not being editable (via the form which is really the only way to cause the contributions to be deleted)