accounts-ui not dependent on global {{currentUser}} handler

Specifically, we don't have an API for defining global handlers right
now (the old Handlebars.registerHelper). We still need to devise
a new pattern for this but in the meanwhile, this is a step towards
getting accounts-ui to work.

packages/accounts-ui-unstyled/login_buttons.js

@@ -10,6 +10,10 @@ Handlebars.registerHelper(
       return new Handlebars.SafeString(Template._loginButtons({align: "left"}));
   });
 
+Template._loginButtons.currentUser = function () {
+  return Meteor.userId();
+};
+
 // shared between dropdown and single mode
 Template._loginButtons.events({
   'click #login-buttons-logout': function() {