Add participants via member number

[bastianjoel]

resolves #4020

Needs OpenSlides/openslides-backend#2586

[Elblinator]

It is currently not possible to create a new user without a member number

Either the payload has to send nothing in the member number (or member number not at all) or the backend has to accept an empty string as the member number

example payload:

[
  {
    "action": "user.create",
    "data": [
      {
        "first_name": "New",
        "last_name": "User",
        "member_number": "",
        "is_active": true,
        "is_physical_person": true,
        "email": "",
        "meeting_id": 1,
        "vote_delegations_from_ids": [],
        "group_ids": [
          1
        ]
      }
    ]
  }
]