Use ST_AsGeoJson directly?

[remicousin]

          I'm not sure why we use `ST_AsBinary` and then use shapely to convert to GeoJSON, instead of just using [`ST_AsGeoJson`](https://postgis.net/docs/ST_AsGeoJSON.html). Possibly because some of it was copy-pasted from fbfmaproom, where shapes are rasterized into the tiles, not sent to the browser as GeoJSON. Something for me to look into.

Originally posted by @aaron-kaplan in #528 (comment)