user notifications

see #2 

- [x] add settings for each user to enable/disable email and/or in-app notifications
- [x] use symfony event dispatcher to emit notification events
- [x] email event subscriber
- [x] in-app event subscriber
- [x] indicate in-app notifications in menu
- [x] user menu shows in-app notifications, user profile is linked from here
- [x] extend markdown syntax to autolink topics by "#"