Strip whitespace around username before validating

This is mostly for when people are logging in. Mobile keyboards
especially like to add a space after the username, which previously
would cause an error unless they manually deleted the trailing space.

tildes/tildes/schemas/user.py

@@ -92,6 +92,17 @@ class UserSchema(Schema):
         if is_breached_password(value):
             raise ValidationError("That password exists in a data breach (see sidebar)")
 
+    @pre_load
+    def prepare_username(self, data: dict) -> dict:
+        """Prepare the username value before it's validated."""
+        if "username" not in data:
+            return data
+
+        # strip any leading/trailing whitespace
+        data["username"] = data["username"].strip()
+
+        return data
+
     @pre_load
     def prepare_email_address(self, data: dict) -> dict:
         """Prepare the email address value before it's validated."""