mojo: Import Mojolicious's error handler debugging page.

I removed some extraneous stuff that would just bloat the repository
like JS syntax highlighters... and also removed the Google Analytics stuff.

This debug handler (or a more specific 404 and/or 500 HTTP status
handler) is required for some of the other Mojolicious error handling
paths to work. For instance, using the default 'helper' "not_found"
tries to find "not_found.development.html.ep" or "not_found.html.ep"
before falling back to "mojo/debug.html.ep", and if it finds nothing it
doesn't render any response. So without a template available for
Mojolicious to use in the page rendering path, Mojolicious just gives up
instead and transmits nothing at all until a timeout occurs (15 seconds
by default).

A barren template could be used but the Mojolicious default debugging
template is actually very good and provides a host of useful debugging
information if it comes to that. So I'd rather base off of this instead.