GUI: Notification overload

[jzbz]

Default for notifications should be that user is notified only if activity takes place in a 1-on-1 chat (DM) or if they are mentioned by name in a group chat (@).

Right now every message in every group is triggering a notification which can be overwhelming and mostly renders the feature useless.

The Element beta notification options might offer decent inspiration for how to implement some granularity here.