Backend implementation of notifications

[JonasMuehlmann]

Before this PR can be merged, the following checkboxes have to be ticked

• This code belongs to the backend and is tested, or tests are not useful
• New functions with more than 5 lines have docstrings
• New code has been integrated with the MessengerService if appropiate
• New code has been integrated with the SignalRService if appropiate
• New functions or classes have been documented in the Entwurfsdokumentation
• Changes introduce no commented out code or Console.Writeline() statements used for debugging

Related issues:

[JayKim94]

@JonasMuehlmann I am not sure if I understood how Mute works. If a user gets 'muted' by another user, the muted user can not send notifications to that user any more correct? Sonst sieht's gut aus

[JonasMuehlmann]

@JonasMuehlmann I am not sure if I understood how Mute works. If a user gets 'muted' by another user, the muted user can not send notifications to that user any more correct? Sonst sieht's gut aus

@JayKim94 Yes, if I add a mute for a specific combination of notification -source, -sender and -event(e.g. you reacting to my messages in the team xyz), I will not get a notification of that event occurring in other views or while I am offline. The only way to find out about it is opening the corresponding view and inferring the event from the content of e.g. the new message in a chat, a new team in my teams navigation, etc.