1) Fixed cognitive complexity issues reported by SonarQube.

2) Fixed code smell 'Add a nested comment explaining why this method is
   empty, or complete the implementation'.

web/migrations/script.py.mako

@@ -30,4 +30,5 @@ def upgrade():
 
 
 def downgrade():
+    # pgAdmin only upgrades, downgrade not implemented.
     ${downgrades if downgrades else "pass"}