Let HST render the page meta data as JSON HTML-comment at end of previewed page

When viewing a page in the channel manager, provide the page's meta data in a JSON-encoded HTML-comment at the end of the returned payload, i.e. after the </html> tag.

This will allow the new channel manager to avoid an additional HEAD request to retrieve the current page's meta data separately. The new channel manager will parse the comments upon navigating to a new page, and update its state accordingly.