Initialized existing members.js data attribute handling

refs https://github.com/TryGhost/members.js/issues/32

- Inits data attribute handling from the copied over members.js file from core
- Uses `window.location.origin` as site url for api calls in data attribute handling

ghost/portal/src/data-attributes.js

@@ -1,3 +1,5 @@
+/* eslint-disable no-console */
+
 function handleDataAttributes({siteUrl}) {
     Array.prototype.forEach.call(document.querySelectorAll('form[data-members-form]'), function (form) {
         let errorEl = form.querySelector('[data-members-error]');

ghost/portal/src/index.js

@@ -3,6 +3,8 @@ import ReactDOM from 'react-dom';
 import './index.css';
 import App from './App';
 
+const handleDataAttributes = require('./data-attributes');
+
 function addRootDiv() {
     const elem = document.createElement('div');
     elem.id = 'ghost-membersjs-root';
@@ -19,6 +21,7 @@ function handleTokenUrl() {
 
 function init() {
     addRootDiv();
+    handleDataAttributes({siteUrl: window.location.origin});
     handleTokenUrl();
     ReactDOM.render(
         <React.StrictMode>