Add a new scenario to test GitHub import

EE MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6383

What does this MR do?

This adds a new Test::Integration::Github test class (qa/qa/scenario/test/integration/github.rb) and one new scenario in this test suite (qa/qa/specs/features/project/import_from_github_spec.rb).

Are there points in the code the reviewer needs to double check?

1. This test uses the gitlab-qa/test-project project path and requires a new GITHUB_ACCESS_TOKEN environment variable.
2. This builds on top of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/19964, which I extracted so that it can be merged sooner and because other MRs that I'm working on depend on it too.

Why was this MR needed?

Because GitHub import is an important piece of GitLab and involves a lot of components including GitHub itself, and many background jobs.

Screenshots Video!

github-import-automated

⚠ THE GITHUB ACCESS TOKEN YOU SEE AT 20/21 SECONDS IN THE VIDEO HAS BEEN RESET, DON'T WORRY! ⚠

Does this MR meet the acceptance criteria?

• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Resolve gitlab-qa#146 (closed).