restore the tokenizer directory when missing

Automatically repopulate the tokenizer/ directory with the PHP stub
and the postgresql module, when the directory is missing. This allows
to switch working directories and in particular run the service
from a different maschine then where it was installed.
Users still need to make sure that .env files are set up correctly
or they will shoot themselves in the foot.

See #2515.

nominatim/db/properties.py

@@ -27,6 +27,9 @@ def get_property(conn, name):
     """ Return the current value of the given propery or None if the property
         is not set.
     """
+    if not conn.table_exists('nominatim_properties'):
+        return None
+
     with conn.cursor() as cur:
         cur.execute('SELECT value FROM nominatim_properties WHERE property = %s',
                     (name, ))