Adding full Article support

This creates a new column in the `statuses` table which keeps track of
activity_pub_type, so in the case of a Note it will be blank (the
default) and it will be a string "Article" if the received remote object
is an AP Article. There is now a bunch of special case code in the
formatters and sanitizers to handle Articles differently, as well as on
the clientside.

app/lib/sanitize_config.rb

@@ -49,6 +49,9 @@ class Sanitize
           'rel' => 'nofollow noopener',
           'target' => '_blank',
         },
+        'span' => {
+          'class' => 'article-type',
+        },
       },
 
       protocols: {