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This PR replaces `osmborder`, which is no longer maintained, with `imposm` mappings and SQL code to generate borders.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`.

.github/workflows/performance.yml

@@ -153,13 +153,12 @@ jobs:
             make start-db
             profile 1_data     make import-data
             profile 2_osm      make import-osm
-            profile 3_borders  make import-borders
 
             if [ -f ../ci_cache/wikidata-cache.json ]; then
               cp ../ci_cache/wikidata-cache.json cache/wikidata-cache.json
             fi
-            profile 4_wikidata make import-wikidata
-            profile 5_sql      make import-sql
+            profile 3_wikidata make import-wikidata
+            profile 4_sql      make import-sql
 
             # Get database total size, in MB
             # Once Makefile has a few more improvements, we can use this approach instead: