tumblr: convert commentary to dtext.

* Convert Tumblr commentary to DText.
* Strip extraneous whitespace in links and blockquotes.
* Add newlines after block elements to ensure they're separated from
  subsequent blocks.

app/logical/sources/strategies/tumblr.rb

@@ -53,6 +53,10 @@ module Sources::Strategies
       end
     end
 
+    def dtext_artist_commentary_desc
+      DText.from_html(artist_commentary_desc).strip
+    end
+
     def image_url
       image_urls.first
     end