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/formatter.rb

@@ -92,6 +92,7 @@ class Formatter
 
   def format_article(text)
     text = text.gsub(/>\n+</, "><")
+    text = "<span class='article-type'>#{text}</span>"
     text.html_safe # rubocop:disable Rails/OutputSafety
   end