After having tested success and failure notifications on test, we want to deploy them on de-wiki. This should not happen earlier than 2 weeks after its deploy on test (i.e. not before August 30th) and only after T143821: Test mention notifications on test cluster has been done.

This task also includes deploying T140928: Find out if mention notifications are used

Background:
This task is based on a wish that received 21 points (#9) in the German Community Wishlist (T139962). Originally, the wish was to receive an information about the sent messages. To help people use mentions in a way that they are sent, we have also included notifications about mention failures, which add the reasons for the failure as well.