City picture ref is no longer stored in database.

Each city has now a subdirectory inside the cities folder.
Update gitignore to ignore all data files.
Changing name in mod-city.php now modifies content dir name.
Country flag ref is no longer stored in database.

Each country has now a subdirectory inside the countries folder.
Changing name in mod-country.php now modifies content dir name.
Adding a flag is now mandatory in add-country.php.