[services] don't hardcode 'users' group for register_user
Let the caller choose which group the user should be in. Related to #3020639
Caveat: this is a possible security hole if untrusted values are allowed.
[server] Refactor Repository.register_user into a CubicWeb service (closes #3020639)
[jcr: move commit to the caller; add item in release notes; fix NameErrors]