Provide local CSS when external CDN fails

Fixes #352

Signed-off-by: Ryan O'Hara <oharar@email.chop.edu>
I think this can be chained like this:

```javascript
var local_bootstrap = document.createElement('link');
    .setAttribute('rel', 'stylesheet');
    .setAttribute('type', 'text/css');
    .setAttribute('href', '/static/stylesheets/css/' + cssStyles[cs]);
```