Now, on OAuth signup form, we create a unique username with random appended string only if there's a conflict.
Previously, this was always happening during the Slack OAuth flow. This has the benefit of preventing username collisions for all OAuth providers.
All usernames should be lowercase, so this generates any username suffix
(in cases of collision) with only lowercase letters. It also removes
vowels to prevent bad 5-letter words from forming.
Ref T712
