Change username validation

src/graphql.rs

@@ -121,7 +121,7 @@ fn is_valid_user_name(username: &str) -> bool {
 
     for c in username[1..].chars() {
         if !(
-            c.is_ascii_alphabetic() && c.is_ascii_lowercase() ||
+            c.is_alphabetic() && c.to_string() == c.to_lowercase().to_string() ||
             c.is_ascii_digit() ||
             "_-.".contains(c)
         ) {