[server] Refactor Repository.register_user into a CubicWeb service (closes #3020639)
[jcr: move commit to the caller; add item in release notes; fix NameErrors]
[migration] set a cnxset before process_script()
Not pretty, but I don't have a better idea right now and this at least
ensures migration scripts get run in the expected environment and can
access sql stuff as before.