[sobjects] Drop custom unique constraint checks in user registration service
This constraints (on unique CWUser's login and EmailAddress's login) are
handled by the backend.
This follows "[hooks/integrity] kill CheckUniqueHook".
Extend test for the ValidationError along the way.
Additional services
===================
In this chapter, we introduce services crossing the *web -
repository - administration* organisation of the first parts of the
CubicWeb book. Those services can be either proper services (like the
undo functionality) or mere *topical cross-sections* across CubicWeb.
.. toctree::
:maxdepth: 2
undo