When a node is overridden to NOT use the active comment source, preserve that setting for new sources

[baisong]

QA todo

1. In App Settings, set the comment source for a given node type (i.e. blog) to any comment source except "No comments".
2. Create a new blog post.
3. On the node edit form, uncheck the box in Comment Settings.
4. Save the node.
5. In App settings, change the comment source to any other source except "No comments".
6. Load the node edit form for the node you just added.
7. Test: In Comment Settings, the new setting is unchecked.