Make can_register_users a property.

Previously this was ambiguous (some implementations had a method,
others had a property), and appears to have mostly been working by
chance rather than anything more subtantial. Add tests which both
assert that this property returns the right values and provide sample
usage. Change the only internal usage to use it as a property.

Change-Id: Iab9d194e388aeb13ce1170a6ce5e303d6844d9de