Use file checksum to detect when a duplicate image is uploaded

[odin-h]

Problem

When uploading an image that has already been uploaded before, the image is added as a duplicate and given a new proof ID.

Proposed solution

Use the file checksum to check if the same file already exists and assign the existing proof ID of that file to the price upload form instead of reuploading the file.

Additional context

Additionally, a message could be displayed informing the user about what happened. To do this, checksums of the original files would probably have to be stored since they won't be the same as the final compressed files.

[raphodn]

Closing in favor of a dedicated issue in the backend : openfoodfacts/open-prices#514