[hooks/integrity] kill CheckUniqueHook
It's the backend's job.
The noticeable consequence (as can be seen in tests updated here) is that
error message are a bit less user-friendly.
.. _cwprops:
The property mecanism
---------------------
.. XXX CWProperty and co
Property API
~~~~~~~~~~~~
.. XXX feed me
Registering and using your own property
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.. XXX feed me