Make validation compatible with the Constraint Validation API

[MarkMcDonald]

Currently, HTMLFormElement.checkValidity() will report false even though the JQuery validate methods validate the form in cases where there are required conditional fields.
https://developer.mozilla.org/en-US/docs/Web/API/HTMLFormElement

Constraint validation still validates fields even when hidden, but should not if disabled so disabling the field as well as hiding may well resolve the issue.