Roadmap backstage enhancements

Summary

This issue lists enhancements that can be made to Roadmap vue app in order to make it more in line with rest of apps at GitLab.

• Break sortEpics method into multiple testable smaller methods.
  • As of now, sortEpics is doing lot of stuff where adding tests for call combinations isn't feasible, as a result we ran into https://gitlab.com/gitlab-org/gitlab-ee/issues/9829 which could've been avoided if we had more concise test cases.
• Convert into a Vuex app and discard eventHub (We've decided to keep eventHub due to simplicity of scroll events handling).
  • Roadmap doesn't include much of a user-interaction as of now and can be converted into Vuex app without much effort, this would be useful once we start adding more user interactions in future releases. Also, it would be a larger effort later if we don't do this now. 🤷