HTMLForm: Suppress HTML5 form validation for non-JS users when needed

HTMLForm: Suppress HTML5 form validation for non-JS users when needed

Our 'hide-if' fields are fundamentally incompatible with HTML5 form
validation attributes. If you have a checkbox field A, and field B
that is required, but hidden if A is unchecked - that's impossible to
express with HTML5 form validation. The only thing you can do is
remove the validation on B (or on the entire form).

The field contents are still validated server-side, just like if the
browser did not support HTML5 forms. The validation is also re-enabled
in JavaScript, since we have extra support for 'hide-if' field that
makes them work.

Change-Id: Ia7ffa76965a7c14af9b6d2db007b6255498398d9