Use ajxp_user_rights to know existing users in listUserTeams

Signed-off-by: Etienne CHAMPETIER <etienne.champetier@fiducial.net>
Rework deleteUser deleteGroup, put logic in AuthService

Deleting a group wasn't calling the hooks user.before_delete
and user.after_delete

Signed-off-by: Etienne CHAMPETIER <etienne.champetier@fiducial.net>