Rebuild every attachment before editing a message

The editor keeps an attachment's content as it finds it, so a hand-written
embed with a url but no href reached the editor with its markup unvalidated,
and a mention saved under Trix's editor carried the generic octet-stream
content type the editor doesn't permit and was dropped on save. Rebuilding
each attachment from its attachable renders the hardened preview partial
and restores the mention content type.

app/models/user/mentionable.rb

@@ -1,6 +1,12 @@
 module User::Mentionable
   include ActionText::Attachable
 
+  MENTION_CONTENT_TYPE = "application/vnd.campfire.mention"
+
+  def attachable_content_type
+    MENTION_CONTENT_TYPE
+  end
+
   def to_attachable_partial_path
     "users/mention"
   end