Create CI/CD-only projects from GitHub

What does this MR do?

CE backport: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/17432

BE: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4567, https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4762

• Create CI/CD-only projects, "GitHub"
  • Re-use existing GitHub import templates and change wording from "Import" to "Connect"
• Other MR for CI/CD-only projects, "Repo by URL", https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4642

After reloading the page or coming back again to connect/import more projects

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

Test with,

• "Personal access token"(PAT) flow
• GitHub integration flow, where you sign in with GitHub and can list repositories automagically
  • To setup the GitHub integration, see https://docs.gitlab.com/ee/integration/github.html

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #3839 (closed)