Attach and detach eProtect functionality as needed

Problem/motivation:

The commerce_vantiv.eprotect.js has code that delegates the checkout form's submit button, but does not detach that functionality e.g. when a vantiv payment gateway form is replaced by another, causing its logic to still apply to the checkout form when it shouldn't.

Proposed resolution:

1. Add the click handler only when the required form element(s) exist.
2. Remove the click handler in drupalBehaviors::detach.

This is tricky behavior but it should be testable. I'll leave that for another issue, though.