Prices images improvement

[monsieurtanuki]

We currently have a minimal proof image management for prices ("select an image").

There are several possible improvements:

1. add the "crop" feature
2. add the "manually erase/mask personal data in the image" feature
3. use a specific "always camera/always gallery/always ask" preference for prices - users may have a different typical usage of camera and gallery for product images and price proof images