Un-vendor CI templates

Per discussions in https://gitlab.com/gitlab-org/gitlab-ce/issues/42265 we agreed that the best thing to do here is to no longer store our templates in the https://gitlab.com/gitlab-org/gitlab-ci-yml repo

As such we should instead just pull them into our repo and manage them there.

This requires a few steps:

• Move the files from the vendor/ directory to somewhere else in gitlab-ce since this word vendor no longer conveys the right meaning anymore
• Ensure the tests we were running in https://gitlab.com/gitlab-org/gitlab-ci-yml/blob/master/verify_templates.rb are still covered in gitlab-ce (but now they're simpler because we just use the parser code in gitlab-ce rather than calling the API)
• Ensure we've copied the latest files for everything from https://gitlab.com/gitlab-org/gitlab-ci-yml
• Deprecate gitlab-org/gitlab-ci-yml and update README to point to the new home of these files
• Archive https://gitlab.com/gitlab-org/gitlab-ci-yml