Tag stats announcements to multiple orgs

https://www.agileplannerapp.com/boards/173808/cards/5818

As a department editor
I want to tag a Stats Announcement to all relevant organisations
So that I can accurately reflect responsibility for Stats Announcements,
  and so the Announcement is referenced from aggregated content for each relevant organisation.

Contains a data migration to populate the join table with existing `StatisticsAnnouncement` and `Organisation` 1-1 associations. I'll follow this up with a migration to drop the `organisation_id` column on `StatisticsAnnouncement`, once the data migration runs successfully on production – https://github.com/alphagov/whitehall/pull/1752
Is this necessary? As far as I can tell, the indexable content won't have changed.