make sure form doesn't make it possible to post to locked topic

app/models.py

@@ -71,6 +71,18 @@ class Users(Model):
                 subscriptions.user_id = ?"""
         return db.query(q, self.id)
 
+    def can_post_to_topic(self, topic):
+        if self.is_guest():
+            return False
+
+        if self.is_mod():
+            return True
+
+        if topic['is_locked']:
+            return False
+
+        return True
+
 
 class Topics(Model):
     table = "topics"