Fix handling of title-less Planet posts
    
    Some platforms can give us a post without a title. When that happens,
    Planet's RSS feed title is "$author" instead of "$author.title", which
    causes our split to blow up. Instead, we check to see if there is a ':',
    and if so split like normal. If not, explicitly set the title to
    "(untitled post)" in order for it to still render.
    
    While I'm at it, update a comment so that it reflects what the code is
    actually doing.
    
    Fixes #9251
    
    Signed-off-by: Ben Cotton <bcotton@fedoraproject.org>