move filling of postcode table to python

move filling of postcode table to python

The Python code now takes care of reading postcodes from placex,
enhancing them with potentially existing external postcodes and
updating location_postcodes accordingly. The initial setup and
updates use exactly the same function.

External postcode handling has been generalized. External postcodes
for any country are now accepted. The format of the external postcode
file has changed. We now expect CSV, potentially gzipped. The
postcodes are no longer saved in the database.