Remove organisation_id from StatisticAnnouncement

https://github.com/alphagov/whitehall/pull/1751

the change above introduces a join table to store
M-M relations between statistics announcements and
organisations. hence this column is not required.
Ebola response topical event slug change.
Merge pull request #1782 from alphagov/ebola-slug-change

Ebola response topical event slug change.

https://govuk.zendesk.com/agent/tickets/850192
Merge pull request #1752 from alphagov/remove-organisation-id-from-statistics-announcements

Remove organisation_id from StatisticAnnouncement