Add listing quantity to transactions

- [x] Add `listing_quantity` to `transactions`
- [x] Take it into use in booking
- [x] Migrate old booking transactions
Sure. What would be the benefit?

I prefer to have very small migrations and I split them aggressively if possible. It's annoying if the migrations fails in the middle and ends up in a state that can not be rolled back, nor migration can not be run again.