Add shortcuts to the new open prices features

[teolemon]

Problem

• add a shortcut to the price validation assistant https://prices.openfoodfacts.org/experiments/price-validation-assistant
• Add a shortcut to the multiple proof addition system: https://prices.openfoodfacts.org/proofs/add/multiple
• add a shortcut to the challenges page: https://prices.openfoodfacts.org/experiments/challenge

[khushi-hura]

hey!

[teolemon]

@khushi-hura @monsieurtanuki implemented the Contribution Assistant part already (there was an older, smaller issue he was assigned on).
You can take inspiration from his PR for the 2 other links.

[khushi-hura]

@teolemon okayy I am on it!

[monsieurtanuki]

Done in #6408.