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 from a file URL (most commonly,
when you've just updated the API specs and want to preview them on
your local machine without spinning up a server).

This change adds a little javascript to change the link href so that
the CSS is loaded via http when the page is served from a file URL. 
(The CSS continues to load as normal via whatever scheme the rest of the page 
was loaded from when the page is served any other way).

Result: much prettier API specs when viewed locally.