Use prettier notification images

Avatars are shown rounded in the main interface, so they should look the same in the notification tray too. On top of that, if the room is a group then show that group's icon when applicable in the bottom right.

Messages

Invites

When the user has no avatar (or it's a direct message) so the icon given is the room's avatar, it shouldn't draw the same icon again in the bottom-right. I've been trying to screenshot it but it's difficult to re-create :-/