SSO uses a special param to username suggester that whitelists a username
due to previous work we amended our lookup logic and started ignoring this
whitelist.
The fix ensures we always respect it, and also improves on the original
implementation that forgot to normalize the username.
Previously username suggester would give up after 100 attempts at getting
a username and fallback to random string.
This amends the logic so we do all the work of figuring out a good username
in SQL and avoids a large amount of queries in cases where a lot of usernames
were used up.
This corrects an issue on sites with large numbers of anon users
We had quite a few cases in core where inputs are being mutated as a side
effect of calling a method.
This handles all the cases where specs caught this.
Mutating inputs makes code harder to reason about. Eg:
```
frog = "frog"
jump(frog)
puts frog
"fly" # ?????
```
This commit is part of a followup commit that adds # frozen_string_literal
to all our specs.
