[RFE] Provide rating notifications and stats to app maintainers

As a dev/maintainer,
- I expect to be informed when my app gets rated. Currently, this is not the case at least for ratings without text (or there exists a bug that prevents notifications for me).
- I also expect transparency on the current status of all ratings (like “35 bad, 167 neutral, 28 good”), ideally with timestamps.

(At least from an app maintainer's POV, these data points do not need to be connected to a user.)

(A similar text was initially written in the [community forum](https://help.nextcloud.com/t/how-are-ratings-in-the-app-store-calculated-and-shouldnt-old-ratings-deteriorate/141254).)