[API] User ownership reassign
For case where a user leaves the organization and her Arvados stuff needs to be given to someone else.
Admin utility script and/or Workbench 2 feature. Use merge API call to assign ownership of departing user's stuff to new user (must be done on each cluster?). In the API server, must allow redirect_to_user_uuid=nil, in which case ApiClientAuthorization (API tokens) and AuthorizedKey (ssh keys) are not migrated, because the user that departed may have still have credentials and shouldn't gain access to other account.