Don't lose formatting in draft comment

When displaying a bubble in the `creating` state, we were wrongly using
the plaintext version of the draft comment. This has the effect of
losing formatting whenever the page is saved and reloaded.

app/models/bubble/messages.rb

@@ -11,7 +11,7 @@ module Bubble::Messages
   end
 
   def draft_comment
-    find_or_build_initial_comment.body_plain_text
+    find_or_build_initial_comment.body.content
   end
 
   def draft_comment=(body)