implement advanced checks for "location.address.Zipcode" for germany

[christian-weiss]

As written here:
freifunk/api.freifunk.net#156
the field "location.address.Zipcode" allows strings without further limits or pattern checks in Freifunk JSON schema. This is to support all countries in the world.

Freifunk Community Registry Quality Report should apply more advanced checks on this field for germany to ensure it is valid german postal code.

Pattern matching, checks against a postal code database and for international postal codes against API from open street map.