normalize utf characters in user names

app/auth/routes.py

@@ -6,7 +6,7 @@ from flask_babel import _
 from app import db, cache
 from app.auth import bp
 from app.auth.forms import LoginForm, RegistrationForm, ResetPasswordRequestForm, ResetPasswordForm
-from app.auth.util import random_token
+from app.auth.util import random_token, normalize_utf
 from app.models import User, utcnow, IpBan
 from app.auth.email import send_password_reset_email, send_welcome_email, send_verification_email
 from app.activitypub.signature import RsaKeys
@@ -95,6 +95,10 @@ def register():
             else:
                 verification_token = random_token(16)
                 form.user_name.data = form.user_name.data.strip()
+                before_normalize = form.user_name.data
+                form.user_name.data = normalize_utf(form.user_name.data)
+                if before_normalize != form.user_name.data:
+                    flash(_('Your username contained special letters so it was changed to %(name)s.', name=form.user_name.data), 'warning')
                 user = User(user_name=form.user_name.data, title=form.user_name.data, email=form.real_email.data,
                             verification_token=verification_token, instance_id=1, ip_address=ip_address(),
                             banned=user_ip_banned() or user_cookie_banned())

app/auth/util.py

@@ -1,7 +1,12 @@
 import random
+from unicodedata import normalize
 
 
 # Return a random string of 6 letter/digits.
 def random_token(length=6) -> str:
     return "".join(
         [random.choice('0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ') for x in range(length)])
+
+
+def normalize_utf(username):
+    return normalize('NFKC', username)