Try merge upstream

**This PR isn't ready for merge yet.**

I'm also not sure if rebase or merge is the better option, so maybe we just take this to make sure our code fits into the official active_merchant gem and then copy all stuff over.

Also, there are methods related to the credit card processing lifecycle which changed in both git trees. I didn't want to remove that code from Shopify completely so I just commented it out. I think this is the main reason that  ~20 tests fail.

- https://github.com/planio-gmbh/active_merchant/blob/try-merge-upstream/lib/active_merchant/billing/gateways/wirecard.rb#L31-L33
- https://github.com/planio-gmbh/active_merchant/blob/try-merge-upstream/lib/active_merchant/billing/gateways/wirecard.rb#L161-L185

- [ ] double-check CreditCard processing lifecycle events / requests
- [ ] make all tests pass
- [ ] add tests for SEPA / Elastic Payments API