Add support of GeoJSON output for list services

Might be needed for document-types vector layers in the UI maps, see https://github.com/c2corg/v6_ui/issues/610

* We don't need all the data provided in the UI's advanced search pages (using the standard document listing API service). Perhaps only document attributes such as document_id, title and some document-type specific attributes such as waypoint_type or elevation.
* We probably don't need to apply filters (except BBOX).
* We might have to return a lot of items, eg. when using a wide BBOX

Then it's probably better and more performant to create dedicated (simplified) services.