Adds a few additional behaviors to add-ons in the subscribe ui

1. setting add-on quantity to zero will remove the add-on
2. setting add-on quantity to a negative value will not calculate a negative total, instead calculating zero contribution to the total
3.  removing an add-on with a quantity less than one, then re-adding that add-on will reset its quantity to 1

Signed-off-by: Christopher Rogers <chrissrogers@gmail.com>
Merge pull request #92 from chrissrogers/feature/quantity-field-improvements

Adds a few additional behaviors to add-ons in the subscribe ui