Proofs: allow anonymous contributions

[raphodn]

Story

If we want to simplify contribution and user onboarding (for instance in "community challenges"), we could allow anonymous users to upload proofs.

• owner ? set to the generic openfoodfacts-contributors user
  • https://world.openfoodfacts.org/contributor/openfoodfacts-contributors
• type ? restrict to only specific proofs such as PRICE_TAG & RECEIPT ? Can be implemented further down the line after we see usage (+ auto-classification in the future will avoid needing to send this field)

[teolemon]

test