Registration doesn't reject KGS/IGS usernames with non-English characters

[0lionelzhang0]

Summary of the Bug

KGS and IGS don't support usernames with non-English characters. The registration form should mark these as "not found".

Steps to Reproduce the Behaviour

i.e.,

1. Sign into https://www.gocongress.org/
2. Add an attendee
3. Scroll down to Blitz Tournament (KGS) or Pair Go Tournament (IGS)
4. Enter username with non-English characters (e.g. 大帅sun or TángJiéHáo)
5. Username is not marked green or red

The Expected Behaviour

Usernames for KGS and IGS with non-English characters are rejected.

Screenshots

Notes

Only 2 attendees have registered with invalid usernames at the moment.