New web folder structure for images

[teolemon]

          @monsieurtanuki @teolemon 

the server still support the old paths, but yes we should update the logic in the Dart package.

/// * 12345678 for barcode 12345678
/// * 123/456/789 for barcode 123456789

That will impact only the EAN8, of course.

Also others, e.g. 9 digit 123456789 is now in 000/012/345/6789. We not only split the EAN8, when we compute paths, we pad all codes with 0s so that they have 13 digits.

Originally posted by @stephanegigandet in openfoodfacts/openfoodfacts-infrastructure#363 (comment)

[monsieurtanuki]

@teolemon already coded in #984, ready for review.