Fixes favicon

18F changed a lot from the original, which is great, and this why there is a large number of changed files in this pr.  The fundamental change is simple and involves copying a transparent png to assets/img/ and pointing the default.html template to that file; e.g.,

```html

 <title>{{ page.title }}</title>
        <meta name="viewport" content="width=device-width">
        <link rel="shortcut icon" href="{{ site.baseurl }}/assets/img/usa_logo.png?v=1.2">
        <link rel="stylesheet" href="{{ site.baseurl }}/assets/css/normalize.css">
        <link rel="stylesheet" href="{{ site.baseurl }}/assets/css/main.css">

```