Resolve "Disable SAML and Bitbucket if OmniAuth is disabled"

What does this MR do?

• Disable SAML and Bitbucket if OmniAuth is disabled
• Unify the way we setup OmniAuth and check if it's enabled or not

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

Did I break anything?

EE: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6501

Why was this MR needed?

Follow how the document is saying. See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20302#note_86116711 for detail.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #48932 (closed)