Usernames can be all numbers, not UIDs

[johrstrom]

#3795 broke an unexpected use case: Usernames that are all numbers. So it seems like this needs to be behind some sort of feature flag so that sites with all numbers as usernames correctly work.

From discourse: https://discourse.openondemand.org/t/usernames-of-only-numbers-can-not-login-to-ood-4-worked-in-3-x/3992