Initial attempt at a friendlier 404 page.

This is a potential fix for #32. I'm not a huge fan of how I'm doing this... I basically "hack" the post details view to also display error pages (in this case, 404s).

One thing in particular that I dislike is the fact that the browser-side navigation system thinks it's only handling either root index views at `/` or post detail views at `/:slug/`, but this commit makes it so that it can be handling *any* URL. Right now that means that ultimately the post details view code is being called, but the slug is not actually a valid slug, e.g. the post details code may be calling out to `/api/post/a/b/c/`, which, while it does end up returning a 404 and triggering the proper 404 view, is still yucky.