The maps break if the geojson's resource is re-uploaded

The problem is that, from #3, even though the user selects the geojson resource, we're saving just its url. We did this for performance reasons. If we saved the resource's id itself, we'd have to request its URL before being able to load the map every time a user tried to see it.

I'm not sure how to fix it (easily).

Start by tracing the geojson resource selection and map loading behavior described in #3, comparing what is saved for the resource URL versus its id. Determine how re-uploading a resource invalidates the saved URL, then verify that maps still load after the resource is replaced without unnecessary URL requests.

Written by the indexing model from the issue text.