New field user member_number

[luisa-beerboom]

See OpenSlides/OpenSlides#6759

• Add a new field member_number to the user model.
• Change user.update and user.create to make it settable, but very restricted (TODO: what permissions?)
• Add as new column to account/participant import
  • Matching should be done based on this column as well
  • Throw error if the given number does not match the username/saml_id/other matching criteria
  • number can only be set via import if it is empty on the user and he is identified through some other matching criteria.

[luisa-beerboom]

I used the same permissions as with the username for now