Only unpublished announcements are indexed

Once an announcement's publication has been published, we do not want it
in the search index as the publication itself will already be in there.
Therefor we only index those that do not have a published publication
associated with them.
Replace rake task with data migration

Now that StatisticsAnnouncements know when they are indexable or not,
we can simply call StatisticsAnnouncements.reindex_all and the correct
things will be put in the index.