Allow quantities for invoice items

Yes, I was just thinking about creating another issue for this. This will be very important for me to actually start using storm for my normal invoicing.

I'm working on #735694: timetracking records not taking fixed price from organisation, project or task..
I've read #301301: Timetracker -> Invoice but can't see any more details about exactly what this is about.

If you are adding quantities, I think adding units would be important at the same time.

I would like the invoice to have the number of hours listed, but when selling materials as a part of a job 1 unit of X or 1 box of X would be desirable.