Add KG/CE to monthly consumption form json export

When exporting the monthly consumption forms in the Management tab, the .json with the drinks and products, should be appended with a list "weighted", and a list "unweighted", for weighted, and per unit products respectively.

The list should consist of the productIDs of the products of that type.

This way, the original products list does not get changed, an the file is still backwards compatible with applications that parse it. The addition is for invoice specifications generated by the SBZ website.

The end of the .json file looks like this:

"65": "Fanta PET 1.5L",
"66": "Coca cola (zero) PET 1.25L",
"67": "Grolsch Rose"
},
"weighted" : [
"1","2","5","9","12", etc."
],
"unweighted" : [
"3","4","6","7","8","10","11",etc"
]
}

Start at the monthly consumption form JSON export in the Management tab and inspect how drinks and products are currently assembled. Keep the original products list unchanged, then add weighted and unweighted productID lists classified by product type; done means the export includes both lists and remains compatible with existing parsers.

Written by the indexing model from the issue text.