Send email also when entry got a rating

[david-ziegler]

Problem

currently we only send emails when entries are created or changed.
Our ratings are the moste relevant feature in Terms of Visibility on our map.
The ratings are very hard to control and moderate, because the system does not send notifications.

Solution

We should also send emails when entries get new ratings. I think this would also be interesting for Regionalpiloten, since we also want to have a good quality of ratings...
Send a notification to subscribers whenever a rating is added or discussed.

Related:

Decouple post-processing from requests for Notifications #166