Making "last updated" on blog posts intentional

[ryanmacklin]

It's bugging me that "last updated" always displays. I'd like us to be able to choose a date that shows for last updated. Having an internal "last updated" in our metadata is fine, but we should separate that with the public one. That way, the public "last updated" can be relied upon for substantive changes, not just "we changed something about how our files work, and the last update autopopulated," or if we need to update a link or correct a typo post-publication.