Allow users to edit dates

[TechShooter]

What

There's no way yet to edit a price's date, other than deleting the price and adding it back

To reproduce the behavior:

https://prices.openfoodfacts.net/app/dashboard/prices

Expected behavior

The date should be editable

Part of

#98

[raphodn]

Thanks for creating the issue ! Linked with #402

The API allows to edit the date, so shouldn't be too hard to add it in the frontend :)
https://prices.openfoodfacts.org/api/docs#/Prices/update_price_api_v1_prices__price_id__patch