Per-download quantities

PR for https://github.com/easydigitaldownloads/Easy-Digital-Downloads/issues/2125 to get the ball rolling.

To do:

- [ ] Checkbox that displays (when quantities are enabled) on each download edit/publish screen that either enables or disables quantities for that particular download. Which should it do? enable it?
- [ ] Get it working when ajax is not enabled. This currently only modifies the ajax call.
- [ ] CSS styling on the input field. should be same size as one at checkout
- [ ] Option (or filter) to enable it to show on the download grid. It would look a lot nicer if it only appeared on single download pages for now, agreed?
- [ ] When the quantity is initially changed, it should retrieve the current value from the cart and allow the customer to update it from the same input field