[16.0][17.0][18.0] fs_image: image_medium is obsolete and image_DDD should be implemented

[jbaudoux]

Module

fs_image

Describe the bug

In the mixin used on product, it implements image_medium that was renamed image_128 in odoo 13.0.
When accessing the image_DDD fields like image_128 or image_256, you don't get the right image.

In my opinion, it's a bug when the module was ported v16.

To Reproduce

Affected versions: 16.0, 17.0, 18.0

Steps to reproduce the behavior:

1. Set an image on the product
2. Install the module fs_image and fs_product_multi_image
3. Change the image of the product
4. Look for example on the website your cart, you see the old product that is in image_128

cc @lmignon @rousseldenis @nicolas-delbovier-acsone @kobros-tech

[nicolas-delbovier-acsone]

By the way, I also encountered this error in shopfloor and tried to fix it with a glue module here: OCA/wms#1214