🐛(back) allow ASCII characters in user sub field

All ASCII characters are allowed in a sub, we change the sub validator
to reflect this.

src/backend/core/validators.py

@@ -0,0 +1,9 @@
+"""Custom validators for the core app."""
+
+from django.core.exceptions import ValidationError
+
+
+def sub_validator(value):
+    """Validate that the sub is ASCII only."""
+    if not value.isascii():
+        raise ValidationError("Enter a valid sub. This value should be ASCII only.")