Replace osmborder with imposm/SQL (#1213)

Fixes #1156
Fixes #810
Fixes #1228

This PR replaces `osmborder`, which is no longer maintained, with `imposm` mappings and SQL code to generate borders.  Key features that were moved into the imposm/SQL layer:
1. Grouping by `osm_id` and aggregating by lowest `admin_level` value so that there's only one copy of ways that are members of multiple relations.
2. Filtering out of point features in boundary relations (typically `admin_centre` and `label` roles).
3. Move disputed boundary detection logic into SQL.

This will increase the database size slightly because of the limits of what imposm can do, as some of the filtering is done in the SQL layer after importing, rather than being done in `osmborder`.

quickstart.sh

@@ -234,14 +234,6 @@ echo "      : The OpenstreetMap data license: https://www.openstreetmap.org/copy
 echo "      : Thank you OpenStreetMap Contributors ! "
 make import-osm
 
-echo " "
-echo "-------------------------------------------------------------------------------------"
-echo "====> : Start importing border ${area} data into PostgreSQL using osmborder"
-echo "      : Source code: https://github.com/pnorman/osmborder"
-echo "      : Data license: http://www.openstreetmap.org/copyright"
-echo "      : Thank you Paul Norman"
-make import-borders
-
 echo " "
 echo "-------------------------------------------------------------------------------------"
 echo "====> : Start importing Wikidata: Wikidata Query Service -> PostgreSQL"