Include the Open Sans font during collabdoc preview generation

I think this is a matter of adding some rules in `oae-preview-processor/static/collabdoc/collabdoc.css`.

Something like:
```css
/* Import the Open Sans font */
@import url('//fonts.googleapis.com/css?family=Open+Sans:300italic,400italic,600italic,700italic,400,300,600,700&subset=latin,cyrillic-ext,latin-ext,greek-ext');


body {
    font-family: 'Open Sans Regular', 'Open Sans', Arial, sans-serif;
}
```