Hooks for data-remote forms don't work with the HTML5 form attribute

HTML5 provides the `form` attribute on `input`, `button` etc. elements. This associates the element with the form whose `id` matches the attribute, rather than the closest ancestor form. See the [MDN documentation on `input`](https://developer.mozilla.org/en-US/docs/HTML/Element/input).

jquery-ujs does not interact properly with the `form` attribute; clicking a submit button associated with a remote form via the `form` attribute should invoke normal UJS remote hooks and submit by AJAX. Instead, the form is submitted as a standard HTTP transaction.