Enforce `publisher.logo` in json metadata

[mjangda]

It's a required attribute per #104. We already add a fallback for wpcom, but should add to the main plugin in the following order:

• Site icon
• Fallback image (WP logo or empty image)

Spec: https://developers.google.com/structured-data/rich-snippets/articles

[idevol]

The same user can customize it, like this:

add_filter('amp_post_metadata', 'xyz_amp_post_metadata');
function xyz_amp_post_metadata($metadata = array()){
    // Type
    $metadata['@type'] = 'NewsArticle';
    // Logo
    $metadata['publisher']['logo'] = array(
        '@type'  => 'ImageObject',
        'url'    => get_bloginfo('stylesheet_directory').'/amp/xyz-logo.png',
        'height' => 60,
        'width'  => 250
    );
    return $metadata;
}

[idevol]

Update the code to the new metadata format.

add_filter('amp_post_template_data', 'xyz_amp_post_template_data');
function xyz_amp_post_template_data($data = array()){
    // Type
    $data['metadata']['@type'] = 'NewsArticle';
    // Logo
    $data['metadata']['publisher']['logo'] = array(
        '@type'  => 'ImageObject',
        'url'    => get_bloginfo('stylesheet_directory').'/amp/xyz-logo.png',
        'height' => 60,
        'width'  => 300
    );
    return $data;
}