Uniquify reserved word usernames on OAuth user creation

What does this MR do?

Uniquify reserved word usernames on OAuth user creation. E.g., a username of "admin" will be converted to "admin1" rather than returning a 422 "Username admin is a reserved name"

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

No

Why was this MR needed?

We use CAS authentication to automatically provision accounts on our instance. An external identity such as admin@gmail.com currently results in a 422 because 'admin' is a reserved word. Rather than require a support ticket to get these accounts created, we can uniquify them with a counter just as we do when two usernames collide.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• 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
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?