Add a pathway to price per kilo

[teolemon]

Problem

If we have quantity to be completed status we should show a button to the edit section for quantity.
This ensures the users can get price per kilo within seconds.

[monsieurtanuki]

Interesting topic already evoked in #5437: fixing incomplete data in OFF, and how to make Prices be aware of it.

[teolemon]

We have the luxury to be a client, so I guess we can query the local database ?

[monsieurtanuki]

We currently use the minimalist product data from Prices:

1. should we systematically download in Prices pages the same product but from OFF?
2. should we systematically override Prices product data with OFF product data?
3. should Prices add "updateProduct" methods in order to avoid product data discrepancies?