Show recent cancelled stats announcements on index

The statics announcement index needs to also include cancelled
announcements which were originally due to be published within the
last month. As some stats announcements (ones without a linked
publicaiton) remain in the index, the filter assumes a from_date of
now if one isn't specified to filter out those old announcements.
Thus, we have to go and get the cancelled announcements between now
and 1 month ago and prepend them to the results.

This only applies if the user has not selected a from_date and
is on page 1.

There are very few cancelled announcements (6 or 7 a year), so we're
only ever likely to increase the first page length by 1 or 2 at any
given time. This may in some cases cause the pagination to incorrectly
count the number of pages as the total number of pages can no longer
be exactly calculated from the total result count / number per page.
The symptom of such an error would be a blank page at the end, which
probably isn't the end of the world.

A cleaner solution would require changes to how stats announcements
indexed and it was decided that this grungy approach was the better
way to go. Sorry.
Unscope SpecialistSector relation to Edition 

Each SpecialistSector belongs to an edition. However, if the edition is 
deleted, requesting the relation returns nil as the default scope for 
the Edition model is to exclude deleted editions.

By overriding the relation to be unscoped, we can still perform 
operations on Editions related to SpecialistSectors regardless of their 
state.

This is based on prior art found in 
https://github.com/alphagov/whitehall/blob/ec83c2ea/app/models/classification_membership.rb#L25
Remove stray ' from "its" in a test
Merge pull request #1692 from alphagov/specialist-sector-relates-to-unscoped-edition

Unscope SpecialistSector relation to Edition
Merge pull request #1693 from alphagov/stray-apostrophe

Remove stray ' from "its" in a test
Refactor StatisticsAnnouncementFilter#get_results
Merge pull request #1673 from alphagov/show-cancelled-stats-announcements

Show recent cancelled stats announcements on index
Merge pull request #1694 from alphagov/duplicate-announcement-cleanup

Script for cleaning up duplicate announcements