New generate-dc-config target, rm QUICKSTART_MIN/MAX_ZOOM (#915)

* Set `MAX_ZOOM` to 7 by default.
* Remove `QUICKSTART_MIN/MAX_ZOOM` - unneeded complexity with two env vars. We can just use `MIN_ZOOM` and `MAX_ZOOM`. See also #261
* Generate dc-config yaml file with a new `make generate-dc-config` step. It will compute BBOX based on the downloaded data file. This step is not needed for planet generation.
* Generate Imposm replication file only when `DIFF_MODE` is `true`. Not needed otherwise. If the data source does not support it, it will throw an error.

.github/workflows/tests.yml

@@ -20,12 +20,9 @@ jobs:
       - name: Run quickstart for a small area
         env:
           area: monaco
-          QUICKSTART_MIN_ZOOM: 0
-          QUICKSTART_MAX_ZOOM: 14
+          MIN_ZOOM: 0
+          MAX_ZOOM: 14
         run: |
-          # For now, change the quickstart values directly in the .env file
-          # TODO: We should probably use env vars instead
-          sed -i 's/QUICKSTART_MAX_ZOOM=7/QUICKSTART_MAX_ZOOM=14/g' .env
           export QUIET=1
           ./quickstart.sh $area