Improve generation of merged postal codes database

[rth]

Currently postal codes are merged from two sources in backend/apps/mapview/make_plz_json_script.py. I suppose some of this script could be simplified if pgeocode supported user defined datasets for missing entries (related https://github.com/symerio/pgeocode/issues/13).

Though in the end if the current approach works, maybe it doesn't need fixing..