Github enterprise support importer

What does this MR do?

This restores support for GitHub enterprise in the new GitHub importer and also fixes the Rake task, which was broken when the old importer code was removed.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/39931

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered