Update existing services to ignore new constant events

[pbrisbin]

Soon, ConstantsCreated worker events will trigger quality events that lack
previous ratings values. This PR makes it so events of this type are ignored by
the current set of services (since they can't handle them as-is).

Alternatively, we could update the services to print a more appropriate message
in this case (e.g. "Foo created as a B"). This would require changes in a few
more places and cause many new notifications to go out to users.

I decided it was better to do the smaller change (at least for now) and just
preserve existing behavior while making it possible to send these new-constant
quality events for the purposes of the FeedService.

Let me know if you'd rather I do the alternative.

[pbrisbin]

We're going to hold off on re-using quality for this purpose. No need to merge this code.