Sort user configuration fields by slug names

Refs https://gitlab.com/arkindex/requests/-/issues/643

The form displaying user configuration should sort alphabetically the fields using the "slug" (the key of each field in the dict).

So in the example from the request, it would give the following order:

      extraction_mode
      image_convert
      line_element_type
      lmwt
      max_deskew_angle
      nb_threads
      scale_x
      scale_y_bottom
      scale_y_top
      timeout
      wip