Fixed links in Portal signup terms

Because we load Portal in an iFrame the anchor tags do nto work
correctly. We've intercepted all clicks on them and manually opened
the URL in a new window instead.

ghost/portal/src/components/pages/SignupPage.js

@@ -519,8 +519,15 @@ class SignupPage extends React.Component {
 
         const className = `gh-portal-signup-terms ${errorClassName}`;
 
+        const interceptAnchorClicks = (e) => {
+            if (e.target.tagName === 'A') {
+                e.preventDefault();
+                window.open(e.target.href, '_blank');
+            }
+        };
+
         return (
-            <div className={className}>
+            <div className={className} onClick={interceptAnchorClicks}>
                 {signupTerms}
             </div>
         );