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
Having a factory for a vanilla join model is a bit of an anti-pattern in my opinion. If there is no scenario under which a model instance will be explicitly created, manipulated or interacted with in the codebase, then this factory should't really need to exist. Most of the tests that rely on it don't really provide us with enough value to justify their existence as they are either testing AR validations or that the association is correctly defined.