Integrate gift card with coupon redemption panel in checkout

Problem/Motivation

Currently commerce_giftcard and commerce_coupon both add very similar panels to the checkout for redemption.

This is needed, if only one of both modules is installed, but if both are installed, this is confusing and not acceptable for end-users to understand:

As end-user you don't care what kind of coupon or gift-card you have, you simply enter it and it should work

Steps to reproduce

Install commerce_giftcard and commerce_coupon and see very similar checkout pane forms, where you'd expect a unique one

Proposed resolution

Detect if both modules are installed in commerce_giftcard and use the form from commerce_coupon to redeem gift cards.

Remaining tasks

User interface changes

API changes

Data model changes