MyAccountFavoritesCtrl: $watch for $rootScope.person.id

Looking up favorites for the authenticated user doesn't make sense
unless we have an authenticated user ;).  Before this commit, if a
user refreshed on:

  #/my-account/favorites

the MyAccountFavoritesCtrl would try and initialize while
$rootScope.person was still unset.  With this commit, we can wait
until the value is initialized by AppCtrl (via /session or
SignInCtrl) before launching the Product query.