Port from CE to EE: Changing OAuth lookup to be case insensitive

What does this MR do?

Makes the OAuth lookup to be case insensitive

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

There are differences between EE and CE related to the model Identity. For example, in EE there is another uid field available.

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 UX
  • Has been reviewed by Frontend
  • 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
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan

What are the relevant issue numbers?

gitlab-org/gitlab-ce#38822