Use WireMock for mocking external services

[php-coder]

Let's use WireMock for mocking of the external services. At this moment, we have TestController that is doing such a job for us but it's limited and it shouldn't be part of the application. It also could be used later if we decide to split the application into smaller pieces (microservices).

• http://wiremock.org/
• https://github.com/automatictester/wiremock-maven-plugin