Always show last updated on posts

It's almost always going to be different than the author date, and we want to be showing the metadata for others.

This is important because there are a number of posts that would've been better as having identical publish and update dates (i.e. back-posted) but this has blocked it.

In that case, we would not see both the dt-published and the dt-updated which then breaks the HTMLProofer Checks.

Closes #513 (closed).