Ability to order, count and issue stock in terms of pack size

Is your feature request related to a problem? Please describe.

Users in a number of countries are used to ordering many medicines in terms of number of 'Catalogue' packs, rather than actual quantity. The Catalogue pack size = mSupply default pack size.

mSupply Mobile (and requisitions handling in mSupply desktop) currently can only handle actual quantities.

Describe the solution you'd like

mSupply Mobile should include Pack size and Packs as a columns in requisitions, stock counting, and customer invoices.
Pack size defaults to default pack size (which clearly needs to sync with the item details from the Primary. Does it do this already?)
For Supplier requisitions:
- User can not edit Pack size.
- User enters (number of) Packs.
- User does not enter Quantity. It is calculated.
For Supplier invoices:
- User can enter / edit Pack size.
- User can enter / edit (number of) Packs.
- User does not enter Quantity. It is calculated. A supplier invoice coming from a requisition will have Pack size, Packs and Quantity already populated. Changing either of Pack size or Packs will result in recalculation of Quantity. And yes, this may result in a different total quantity than what was on the supplying store's CI. User should be warned before completing this action if it will result in a change in total quantity than what was on the supplying store's CI.
For stocktakes:
- User can enter / edit Pack size for new batch lines, but not for existing lines. (though this would be a really cool feature if we could allow it - maybe save for another feature request!)
- User can enter / edit (number of) Packs.
- User does not enter Quantity. It is calculated.
For customer requisitions and invoices:
- Each batch's Pack size will appear but can't be edited by the user (as with desktop)
- User can enter / edit (number of) Packs.
- User does not enter Quantity. It is calculated.

Describe alternatives you've considered

No real alternative that I can think of.

Additional context

mSupply Mobile requisitions with pack size will need to play nicely with other mobile stores and with Desktop requisition handling which also needs this feature - refer https://github.com/sussol/msupply/issues/4027

Possibly both desktop and mobile need to be upgraded simultaneously before they sync to each other. Alternatively, requisitions / CIs are processed as actual quantities and then default pack sizes are 'applied' at the other store... Sounds messy and error prone to me.

msupply-foundation / mobile