Make variations script re-usable

At the moment, the variations script is partially re-usable since it can be dynamically attached to a DOM element. To make the variations script 100% re-usable on dynamically loaded (and possibly multiple) ```variations_form``` elements, all references to input names must be removed, since those inputs may have different (unique) names when submitting data for multiple variable products inside the same ```form```.

Therefore,

- the variation_id input field must be referenced by class
- quantity input fields must be referenced by class
- attribute names must be referenced by class or -- probably better, as done -- a data attribute.