Fix two errors in two internal templates

Fix two errors in two internal templates

 * template: _internal/_default/opengraph.html:39: unexpected EOF
 * template: _internal/_default/schema.html:15: unexpected {{end}}

Also change the DateTime inside these templates to ISO 8601 format,
and skip <meta itemprop="datePublished"> if `publishdate` is not set.

Perhaps it would be a good idea to expose `func (Time) IsZero`
to our templates?  :-)