Deprecate summary elements in (Geo)JSON response formats

There are some major inconsistencies in all response formats which don't come unnoticed (#734). We need to redesign a clean solution and deploy at some point as API v3.

Back in February 2020 we agreed in our internal discussions that the `summary` element in JSON/GeoJSON is redundant. The fields within the summary should be moved to top-level and the summary-element itself removed in v3.

Any newly introduced fields in the response (such as e.g. `departure` and `arrival`) should be already put on the top level. We will start transitioning by duplicating fields and deprecating the bad positions.

The issue names no files, tests, or entry points. Start by locating the JSON/GeoJSON response-format implementation and API versioning path; done means summary fields are transitioned to top-level positions and the summary element is removed in API v3, with newly introduced fields kept at the top level.

Written by the indexing model from the issue text.