Switch from Dredd tests to PHPUnit-based contract testing

[dafeder]

Dredd tests are no longer feasible for our complete OpenAPI 3 spec. We should switch to a more flexible framework for contract testing that supports v3. Most convenient would probably be something that integrates with PHPUnit. Current favorite option would be something build on openapi-validator.

Read more about that library here.

If we can implement this completely, it may have the added benefit of being able to replace all or most of our existing functional tests.