Modify newuser process to prevent mail redirection errors

[BNH440]

Prevent uppercase letters in usernames so postfix doesn't fail to redirect mail (due to it changing all emails to lowercase before checking users). Only was an issue for PB redirects because that's all we use postfix for right now, may be a larger issue once we do mailman migration.

See conversation here for details: https://discord.com/channels/784902200102354985/1300627201607204884/1332229183404904449

Here's the temp fix I did

/etc/aliases/

# (added 1/23/25 by blakeh) to fix pb mailing list forwarding with postfix because it breaks with capitalized usernames
n.drea: a_tsai@berkeley.edu
potatoaad: matthew_qi@berkeley.edu