Provide reverse-lookup on views for Licenses -> Subscriptions

Here's a patch that does this. It could be pretty easily updated to also pull in a Subscription to an Order using reverse lookup. I heavily copied the relationship plugin from Webform Views, which, as I said in the description, would allow us to do the same thing with orders, if desired.

The last submitted patch, 2: commerce_recurring-relationship-3391785-2.patch, failed testing. View results →