[IMP] base,website*: do not store country flags.

1) Avoid the storage of all country flags as ir_attachment (230+
ir_attachment in a new db) to reduce the filestore of databases.

The country flags are nearly static and not expected to be modified on
Odoo instances.

This change is based on the new "image_url" widget logic (see previous commits).

2) Extend the flag coverage for countries

Add the missing country flags & specify a mapping to provide flags
for overseas administrated countries/territories.