Add a special view to add prices for products without barcodes

[teolemon]

What

• Add a special view to add prices for products without barcodes
• It should let you enter a Category, an Origin, a boolean Organic/Not Organic, A price, A location, A photo proof
• https://prices.openfoodfacts.org/app/add/single

Entry point

• We might shove it in DEV mode or in settings. In the future, we might intercept search queries on specific categories, and add a promo on top of search results

Temporary alternative solution

• Create a link to the web version, with "Add a price for a product without barcode", linking to : https://prices.openfoodfacts.org/app/add/single

Part of

• Open Prices (tracker) #5072

[teolemon]

Note to @raphodn: it would be nice to be able to pre-populate:
https://prices.openfoodfacts.org/app/add/single?nobarcode=true