[New: openFATE 310657] New user registration improvments.

Feature added by: Adrian Schröter (adrianSuSE) Feature #310657, revision 1 Title: New user registration improvments. Buildservice: Done Priority Requester: Important Requested by: Adrian Schröter (adriansuse) Description: webui shall offer to use the existing api user registration call. admins shall also be able to limit user registration to * deny in general * allow, but require admin approval. -- openSUSE Feature: https://features.opensuse.org/310657