Closes#904
* Make all data-related targets like `download*`, `import-osm`, `import-borders`, and `generate-tiles` into `area`-aware -- making it possible for multiple data files to coexist inside the `./data` dir.
* Add `make download area=... [url=...]` command to automatically download any kind of area by checking Geofabrik, BBBike, and OSM.fr, optionally from a custom URL. Supports `area=planet` too.
* Do not re-download area with `make download-*` if it already exists.
* Automatically rename `<area>-latest.osm.pbf` into `<area>.osm.pbf`
* If `area=...` parameter is not given to `make`, see if there is exactly one `*.osm.pbf` file, and if so, use `*` as the `area`.
* Configure many variables in the .env file, overriding the defaults in tools
* If `<area>.osm.pbf` exists, but `<area>.dc-config.pbf` is missing, generate it using `download-osm make-dc` command.
Also:
* closes#614
* closes#647
* partially addresses #261
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