Allow Bootstrap CSS to load when API specs are run from disk

Currently the Bootstrap CSS is loaded using a scheme-less URL, so
that it loads properly when the page is requested via HTTP and HTTPS.

This doesn't work when the page is accessed as a file (most commonly,
when you've just updated the API specs and want to preview them on
your local machine).

This change adds a little javascript to change the media href to load
the CSS via http when the page is served from a file URL.