Migrate OSM cache to localForage/IndexedDB

We don't want to hit limits with localStorage, and it's non-blocking

tests/acceptance/contributions-test.js

@@ -20,7 +20,7 @@ class MockOsmService extends Service {
   }
 
   getCachedOsmObject() {
-    return null;
+    return Promise.resolve(null);
   }
 }