Refactor build

Replaces `download_natural_earth_data.sh` with `make` targets and fully respects `DATABASE_URL` (from either the environment or a foreman-style `.env` file).

To bootstrap an extract:

```bash
echo postgres:///toner_sf # use make .env to use the default db: toner
make install db/sf # or anything in PLACES
```

This also simplifies the Docker data volume.  To build one of those:

```bash
make db/shared
docker build shp/
```

`db/shared` should still be split into `postgres` and `shapefile` targets to facilitate this use-case.

/cc @almccon