[EE] Refactor OmniauthCallbacksController to remove duplication

What does this MR do?

EE MR for https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16694

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

Why was this MR needed?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• EE specific content should be in the top level /ee folder
• Squashed related commits together
• End-to-end tests pass (package-qa manual pipeline job)

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