Announce the suggestion list to screen readers

The invitee input had no combobox semantics: assistive
technology could not tell that the widget owns a listbox with
suggestions. Mark the input as role="combobox" with
aria-autocomplete="list" and an initial aria-expanded="false",
point aria-controls at the list and give the JS-built list a
matching id so the reference resolves.

js/invitees.js

@@ -30,6 +30,7 @@
   }
 
   var list = document.createElement('ul');
+  list.id = 'atocal-suggestions';
   list.className = 'atocal-suggestions';
   list.hidden = true;
   list.setAttribute('role', 'listbox');