Notificacations for simple @-mentioning in comments

(WIP) notify user when mentioned in comments

Fix doc, and create absolute URL for as notification link.

PSR-4 compatibility changes

also move notification creation to comments app

Do not notify yourself

unit test for controller and application

smaller fixes

- translatable app name
- remove doubles in mention array
- micro perf optimization
- display name: special label for deleted users, keep user id for users that could not be fetched from userManager

Comment Notification-Listener Unit Test

fix email adresses

remove notification when triggering comment was deleted

add and adjust tests

add missing @license tags

simplify NotificationsController registration

appinfo simplification, php docs

make string easier to translate

adjust test

replace dispatcher-based listeners with a registration method and interface

safer to not pass optional data parameter to setSubject for marking as processed. ID and mention suffices

Signed-off-by: Arthur Schiwon <blizzz@arthur-schiwon.de>

update comment

Signed-off-by: Arthur Schiwon <blizzz@arthur-schiwon.de>