Remove user profile picture from system menu and replace with `avatar-default-symbolic`

The user picture in the system menu is really small - too small to make out really. This will be particularly bad with the new generated avatar images. It's also square, and we're making all the other user pictures round.

I think we should probably just remove the image and use the avatar-default-symbolic icon instead. This will be nicer, since it will be consistent with the other menu items.