cubicweb-registration #3020639 Refactor register_user into a CubicWeb service [validation pending]

Register_user (Repository method) should be called as a service, in order to allow to modify it without monkey patch. In particular, it may be useful to change the default user group.

prioritynormal
typebug
done in0.6.0
load1.000
load left0.000
closed by#350ee3cc2f3f [views] Use new RegisterUserService instead of the register_user method
patch[views] Use new RegisterUserService instead of the register_user method [applied]