Hide comments

Based on #9. This will hide comments for all administration screens, including the Comments screen and Dashboard widget. It will also remove Action Scheduler comments from RSS feeds.

It relies on WooCommerce to [join the posts table](https://github.com/woothemes/woocommerce/blob/c981bb56ca6da5d8eca0864fd3260cf8965698fd/includes/class-wc-comments.php#L48:L66).