Add a JH dedicated CI configuration file

We'll change the setting of the default CI configuration file to jh/.gitlab-ci.yml via:

This way we can easily extend the CI configuration for JH without changing the original CI configuration.

@memorycancel Do you think you know how to configure GitLab CI? Do you understand what I was talking about? Let me know if you have any questions, and I'll help you build it up.

/cc @qianzhangxa @kwiebers

The following discussion from !3 (merged) should be addressed:

• @godfat-gitlab started a discussion: (+2 comments)

  For now we can reuse these rules and definitions, but at some point we might want to define its own.

  Actually, since we decided that we no longer want to commit JH related things into the canonical repository, now we can't do this directly. We'll need to figure out a way to extend the CI configuration if jh/ directory exists.

  I am not sure if there's a good way to do this, but to ease how we move forward, we'll need to create files like jh/.gitlab/ci/rails.gitlab-ci.yml and put these definitions over there, and find a way to extend it from there.

  Please puts these changes into jh/.gitlab/ci/rails.gitlab-ci.yml and use https://docs.gitlab.com/ee/ci/yaml/includes.html to include these definitions. /cc @kwiebers as this is another difficulty with a separated repository.