Add article and content footer components

[choldgraf]

This adds the same components / __inner pattern to the article-footer and the content-footer that we've implemented for the other major sections. It doesn't change any end-user behavior, other than these sections now being configurable. Both of them will stack vertically, and there's just a single list of components for each (rather than a start/end as some other sections have). That seemed reasonable given that these are pretty simple sections.

Here's our article layout now, you can see the sections w/ components that are available for each:

• I think this also closes [discussion] Consider modularity of different page components #330 as now all of our theme sections will have modular component functionality