Custom ArticleFormSet for more sensible Article sortorder, sortorder hidden by default

[dokterbob]

In past versions, the sortorder used to be set using a (global, for all Articles) default. In f6c0180 as part of #103 this was changed to forcibly overriding it on save().

This patch dynamically creates unique initial values per Message while still creating new values when the default is not filled in (as is the case when adding new Articles using JS from the Admin).

It also hides the sortorder by default into the 'Optional' fields, as this is secundary with regards to the title and message, on the same level as the URL and image.