Tie in with Nextcloud notifications

[Weasy666]

It would be really nice, if Element could tie in with the notification system of Nextcloud, so that notifications of new messages or mentions get listed in the bell/notification dropdown.
I don't know if that is technically feasible, because i think that Element would somehow need to run in the background to receive messages or you would maybe need to use the matrix-js-sdk to receive matrix message notifications which then are send to Nextclouds notification system.

[kescherCode]

If this is added, users should be able to opt-out, as some users (such as myself) already have other push systems in place.
Additionally, the server admin should be able to enable or disable this feature by default or force-disable it, since it may also significantly increase load of the server the Nextcloud instance is running on, especially in a multi-user environment.