Prevent avatar errors in console

[PVince81]

Currently we rely on the server returning a 404 to confirm that no avatar exists.
This pollutes the logs, both on the browser console but also on the server side.

We should find a better way to achieve this.
So far I don't think the server has a way to detect if another user has an avatar set before requesting it.

[pascalwengerter]

Will be closed with #5077, closed as duplicate in favor of more recent and better documented issue