Resolve "A lot of constants redefinition warnings"

What does this MR do?

• Fix rake lint:all so that it would print the warnings even if everything passed
• Eliminate Ruby warnings
• Unify config/initializers/omniauth.rb for CE and EE

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

EE: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5927

Why was this MR needed?

We hate warnings

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer

What are the relevant issue numbers?

Closes #46481 (closed)