Add a "Help complete serving size" button if nutrition is inputed but serving size is not

[teolemon]

Why

• Serving size: don't display the dropdown if no serving size or serving size = 100g #1137

What

• Add a "Help complete serving size" button if nutrition is inputed but serving size is not
• We should open a bug server side for a new boolean "Serving size is not mentioned on the packaging"
• Blocked on Add the ability to input the serving size (monolingual free text) #2522

Why

• That way we'll be able to display the dropdown, and more generally be able to do useful things with the portion size.

Part of

• Road to Scores (tracker) #379

[monsieurtanuki]

@teolemon You seem to be talking about the product page ("dropdown"), but in the nutrition page I guess we could also put an ElevatedButton (again!) for the serving size when it's not populated.