Import users from a TSV file.

def acp_users_import():
    """Import users from a TSV file."""
    if not current_user.is_admin:
        return error("Not authorized to edit users.", 401)
    if not db:
        return error('The ACP is not available in single-user mode.', 500)

    form = UserImportForm()
    if not form.validate():
        return error("Bad Request", 400)
    fh = request.files['tsv'].stream
    tsv = fh.read()
    return tsv