Set copyright date to modified date of latest article
Set copyright date to modified date of latest article

If this happens to be available, i.e. if the article
has a 'modified' date.

Cherry picked from commit
getpelican/pelican-themes@7b9a52f9bbd38f3b37217f665c9f5654b4b9be1f.

StevenMaude/pelican-bootstrap3-sm,StevenMaude/pelican-bootstrap3-sm