In production, it's helpful to use content addressing to guarantee cache busting
when publishing changes.

However, in development, because the server serves a static copy of the
index.html, that means that changing the Javascript bundle or CSS requires an
otherwise-unnecessary restart of the server.

This change makes the CSS and JS assets use fixed filenames so that a server
restart isn't needed.

Downside of this approach is there's now a dev/prod disparity and the config is
more spread out between files. The positive is we don't need to modify the
semantics of the server at all.