Add support for canonical urls

Denser should support canonical URLs to indicate the root source of the content. It should use canonical_url if found in json_metadata or build an appropriate link from the app property from json_metadata.

Example post with custom canonical_url: https://blog.openhive.network/hive-139531/@engrave/introducing-dblog-feature-hide-unwanted-comments-under-your-blog-articles

It should add canonical link rel as on condenser: