WIP: Refactor OmniauthCallbacksController to remove duplication

What does this MR do?

Begin refactoring OmniauthCallbacksController, first by introducing case;when statements to replace duplicated code. Next we can probably replace those with new objects.

Why was this MR needed?

Much duplication.

I'll be adding a new :per_group_saml provider with different logic in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4076 and didn't want to duplicate things yet again.

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
• End-to-end tests pass (package-qa manual pipeline job)

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