Task: As a user, I want to receive a notification about my mentions that were sent out successfully.

The notification should contain the page where I mentioned people as well as the user names of all sent out mentions. The message should be as mentioned in T136326.

Background: On the German speaking Community Wishlist, it was proposed to be notified which mentions were sent out (as there are multiple reasons why they sometimes do not get sent out). This proposal received 22 points (#9) in the German Community Wishlist[1]. In addition to successful mentions, we figured that it might also make sense to report back, why unsuccessful mentions were not sent.

[1] Link to wish (in German): https://de.wikipedia.org/wiki/Wikipedia:Umfragen/Technische_W%C3%BCnsche_2015/Topw%C3%BCnsche#Bei_Speichern_Hinweis_.C3.BCber_abgegebene_Benachrichtigungen_.5BUmfrage_2015.5D