Create dev mode Gitlab stubbed response service

When we first prototyped this API, we had a stubbed service that mocked data incoming so that we could develop without binding to expensive/difficult-to-configure services. While initially, this was for Github, to better support development we should add a new stubbed service for development. Testing with Gitlab enabled would require the setup of services that are frequently torn down and have a nasty habit of consuming gross amounts of system resources even when idle. Add to this the fact that only 2 people have access to the real file that this functionality is based on (none of which are on the development team), and it adds a high burden to testing/development.