Added --no-superuser flag

To allow import into already existing databases.
Refs: #2719

nominatim/clicmd/args.py

@@ -87,6 +87,7 @@ class NominatimArgs:
     offline: bool
     ignore_errors: bool
     index_noanalyse: bool
+    no_superuser: bool
 
     # Arguments to 'index'
     boundaries_only: bool