Implement AJAX vs regular form distinction.

formalize() now returns a StandardFormalityForm, while
ajaxFormalize() has been added which returns an
AjaxFormalityForm. The AjaxFormalityForm binds using
ajaxOnSubmit, and expects success and failure handlers to
return JsCmds.

Before, we always used ajaxOnSubmit, and success/failure
handlers were expected to return Unit, so there was no way
to send back down an AJAX response. That is now fixed.

Also introduced is a base FormalityForm class, which
provides a unified binder() function for the two subclasses.
Switch example to ajaxFormalize, get the response on the client.