Set ACL when creating URL and Tag.

By setting an ACL we block users from modifying the URL and Tag objects after
they have been created.
Update ACL for Url and Tag classes.

Modified in Parse so Url and Tag object only can be created by master user,
otherwise an user could create a Tag object with lean ACL, make other users
start using the Tag and then change the name of the tag.

Use master key when creating Url and Tag objects to distinguish us from an
ordinary user.
Add link for logging in as guest user

Also rename guest-user to 'Sandra'.