robustness to _ in names

app.py

@@ -110,7 +110,8 @@ def add_solution_cbk(
     # TODO: extract stable_id robustly, if possible.
     if not profile.username:
         return styled_error("Could not retrieve username. Please try signing in again.")
-    user_id = profile.username
+    # We rely on underscores as separators in submission ID, replace it with "-".
+    user_id = profile.username.replace("_", "-")
 
     try:
         # Validating the submission file.