Notifications in Front-End

Not everybody will use e-mails, making most notifications useless. I think it's best to add notifications in the front-end.

The table already exists (and I checked, no difference from 5.6 to 7), so only 2 things need to be done:
- Notifications **not** triggered by user, add ``database`` to ``via``
- Create something in the navbar so users can actually read the notifications