Add support for using markup in news forms

But keeping support for creating news items using HTML even if in settings
MARKUP_FILTER is set because in pootle_autonotices/signals.py the news items
are created using only HTML.
Add South upgrade path for pootle_notifications: add field message_html
Email news using HTML version
Use the HTML version of the news items for RSS
Show sucessfully created notices using its HTML version
Add support for previewing news before publishing them