Add proper sanitization to `largo_meta_box_save()`

`largo_meta_box_save()` is missing proper sanitization:

```
$mydata = array(
		'_wp_post_template' => $_POST['_wp_post_template'],
		'custom_sidebar' 	=> $_POST['custom_sidebar'],
		'largo_byline_text' => $_POST['largo_byline_text'],
		'largo_byline_link' => $_POST['largo_byline_link'],
		'youtube_url' 		=> $_POST['youtube_url']
	);
```