Slave zones should be read-only in the UI

[zi0r]

If you add a slave zone (using pdnsutil create-slave-zone...) the zone properly shows up in dns-ui; however, it displays the delete/add buttons, etc. Which could confuse a user into thinking that this might be an appropriate place to modify that zone.

For zones of type slave, these buttons should be disabled and dns-ui should not give any indications that it is allowed to try to modify any of the records for that zone.