Add GitHub enterprise support to the GitHub Rake task and better handle rate limiting being disabled

What does this MR do?

This adds GitHub enterprise support to the GitHub import Rake task, and fixes the HTTP client so it can handle rate limiting being disabled for EE installations.

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