fix devhub ratings success msg for public apps (bug 950948)

Rewrite the logic for showing the success message for content ratings. 

    Ratings can be created via IARC pinging our API.
    Thus we can't display a success message via the standard POST/req/res.
    To workaround, we stored app's rating's `modified` from edit page.
    When hitting back to the ratings summary page, calc what msg to show.

There are two messages to show:

- message A (app completion): "Congrats, your app is complete and is now being reviewed."
- message B (content ratings saved): "Content ratings successfully saved."

Here are the cases on which messages to show:

1. if app's status upgrades to PENDING when going from edit -> summary page, show message A.
2. else if content ratings were updated when going from edit -> summary page, show message B.