Request export of a single user's data for the specified site_group. All the user's data within the site group will be exported based on the thirdparty_id and thirdparty_provider passed in. The user must belong to the site group, and the caller must outrank them in the section tree (a global admin may act on anyone except themselves); otherwise the request is refused with 403 and nothing is requested. If that cannot be confirmed, the request is refused with 503.