Re-index statistics announcements on org changes

When an organisation changes its slug we also need to re-index the
statistics announcements as they store the organisation slugs in the
search index. They are like editions in many ways but aren't actually a
type of edition so weren't picked up the first time around.

This was noticed because some Ofqual announcements aren't listed
in the right index: https://www.pivotaltracker.com/story/show/84297826