Add disqus identifier

Currently, if the `site_url` is different from `root.url`, comments on thread pages will still work, but comment counts on the homepage will not work.

This change fixes this issue by adding the canonical URL in the `data-disqus-identifier` attribute.  If the post has a `meta.disqus_id` field this will be used to override this identifier instead.  [According to the documentation](http://docs.disqus.com/developers/universal/#comment-count) using the disqus identifier is the preferred method of identifying posts.