Draft: Only show "Last updated" when we have a valid date

There are a few pages, such as the search page, where the page template is used, but where we don't have a valid "last updated" date stamp to use. In these instances we seem to end up with a date of "0001-01-01". Explictly avoid adding the "last updated" element if we have such a date.

Whilst we're here, use the ISO 8601 date format to avoid confusion between the US and English date formats.

Signed-off-by: Martyn Welch martyn.welch@collabora.com