[server] Refactor Repository.register_user into a CubicWeb service (closes #3020639)
[jcr: move commit to the caller; add item in release notes; fix NameErrors]
ifnotrql('CWConstraintType X WHERE X name "RQLUniqueConstraint"',ask_confirm=False):rql('INSERT CWConstraintType X: X name "RQLUniqueConstraint"',ask_confirm=False)