Fix fallback when image loading fails

With the new avatar endpoint there is no difference between unknown
users and errors when generating the placeholder avatar. Therefore the
avatar function will now show the old placeholder if both a user and
displayname was given as parameters.

In case there is no displayname provided we cannot build the proper
placeholder so the unknown user placeholder is shown.

The displayname is not required for the avatar anymore, so we can
get rid of the old code path for placeholders.

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>