Add ajaxOnSubmit to SHtml

`ajaxOnSubmit` should be used similarly to `onSubmit`/`onSubmitUnit`, to
add proper attributes for AJAX submission to a submit `input` or `button` element
without changing the element itself. It should take a `()=>JsCmd` like `ajaxSubmit`
and transform the element it's applied to by adding the right `name` and `onclick`
attributes (the same ones that `ajaxSubmit` puts on the element it generates).