[RFE] Nudge app raters to give a reason for their rating

While I see the benefit of allowing for ratings without text (“stars only”), I propose to nudge people (probably in the "Thank you" page) to leave a reason on why they chose this rating, or even urge them to comment on what should be improved if the rating is not “good”.

(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).)