Allow authored (or 'bylined') articles

This PR allows the addition of authored articles through a new type of
Speech. This allows articles originally published in news publications
or on the internet to be additionally posted to department sites.

After discussion with @heathd, we decided that having authored articles
as a type of Speech isn't a brilliant abstraction, but it doesn't make
the current stretching of the definition of "speech" any worse than it
already is, so we're going to stick with it for now.

https://www.pivotaltracker.com/story/show/46394133