Incorporate backlinks into generated static HTML

[cdata]

As of #51 we will have a basic facility for rendering spheres as HTML. But, we don't yet include backlinks in the rendered output.

Backlinks are a special case that probably should not be rendered as static HTML. Source content is rendered in a highly cache-able way, but that's because it is static for a given revision. Backlinks to any given source content, on the other hand, may change with any given revision of the sphere (even if the source content does not change). It's likely that the best way to implement backlinks will be some kind of dynamic JS widget that appears at the bottom of any given content and queries for static JSON that lives at an updating location within the file tree.