[Avatars] JS should not load same avatar twice

[Avatars] JS should not load same avatar twice

Old code first dit an ajax request to the avatar. Then a new image
object with the same src was created and since we do not cache avatars
yet :(  this resulted in 2 sequential requests to the exact same URL

Now if you set the displayname it will first set the placeholder and
then load the avatar in the background. Only once this time!