Adjusting public display of statuses to look similar to logged-in UI,

fix #361 with rich OEmbed display via iframe, fix #237 by hiding sensitive
content behind a spoiler on public pages

app/views/stream_entries/embed.html.haml

@@ -1,2 +1,2 @@
 .activity-stream.activity-stream-headless
-  = render partial: @type, locals: { @type.to_sym => @stream_entry.activity }
+  = render partial: @type, locals: { @type.to_sym => @stream_entry.activity, centered: true }