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

[ngokevin]

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.

[ngokevin]

@mstriemer r?

[mstriemer]

I've never been through this IARC ratings flow but this looks like you could only process one application at a time. If it is something that takes some time I could see people going through a few apps at the same time in multiple tabs. This may or may not be an issue, I am not sure.

[ngokevin]

It'd be a small edge case with a not too devastating effect (the developer gets no success message).

I'll attempt to key it by app ID, shouldn't be too bad.

[mstriemer]

r+wc