Custom 404 page for keybase.pub

Hello, would you mind adding support for custom 404 page for `.keybase.pub` If we place a `404.html` in the root of `keybase.pub//`, our public website should serve this as a "404 Page not found" page.

I see that when an item/url is not found, the server generates a html with empty `head`:
```

Not Found

```
Is it possible to insert following content instead in the `head` when an item/url not found?
```

```
Many thanks!

Start by locating the keybase.pub handling for missing items and URLs. Compare the current generated “Not Found” response with the requested per-user 404.html behavior and determine whether serving the file or redirecting to it matches the request. Done means a missing URL under .keybase.pub uses that user’s root 404.html when present.

Written by the indexing model from the issue text.