Reduce the requests for Avatar images

[sbaechler]

Currently, the Avatar component does a Ajax request to check if the avatar image is available and to decide if it should show the image or the fallback.

Right now there is no caching of any kind. So when you do a navigation, the request is sent again. This is especially problematic if you have a list of avatars.

I think it would be helpful if we could check if the image was in the browser cache before doing the request.