server: implement json api layer

[sripwoud]

Originally in pseXperiments#5

Instead of serving directly pre generated html documents, we want to serve the DB data as json and let the client web app build the pages.

Probably not too complicated as the handlers logic are already implemented.
We need to duplicate the handlers (add new endpoints to e.g /api/v1/*) and return json instead of rinja templates. E.g here, use of rinja::into_response.

keeping the existing html endpoints and adding new json endpoints (e.g inn and /api/v1/inn) will allow for a smoother transition to the new front end.

The existing API seems to be documented as comments/docstrings already in each handler file.