Allow refunding and cancelling subscriptions when refunding invoice

[Stiofan]

As titled, this is especially required for Stripe and Paypal (at first)

When refunding an invoice if the gateway supports it, it should show two checkboxes

1. Refund payment in [paypa/stripe]
2. Cancel Subscription

These actions should refund the payment in the gateway and cancel the sub, and also adjust the sub in GP.
There should be private notes added to the invoice for these calls.

[pol76]

Would be great to have option for partial refunds too!

[1naveengiri]

https://secure.helpscout.net/conversation/2753152127/62342?viewId=3812058