Surface latest_public_email as a template tag for the modern archive view

[jmduke]

(It's even already in the codepath since we have it for the classic archive template, we just should surface it.)

[jmduke]

This is actually going to be a hell of a scope creep, because it means we shift the description from being non-templatized at all to being templatized, and it's used in a number of places. We should still do it, but it's not the trivial change I was expecting.