Clients like mobile add "-medium.png" to the url to get the
medium avatar if the avatar_url field of user object is set
to some string other than the gravatar URL.
This commit adds a symlink to offer the unknown user avatar at
unknown-user-avatar-medium.png as well so that mobile clients
can correctly render the medium avatars for inaccessible users.
Fixes#28071.
Sahil Batra