admin:statistics_announcements/index handles nil org

Users without an organisation viewing the statistics announcement index page
should not raise an exception.
Remove duplicate method implementation that exists on inherited class

Found this method while trying to understand how uploads work. It
looks like it's a clear duplicate of the same code that exists in the
`Row::attachments_from_columns` implementation[1].

[1] https://github.com/alphagov/whitehall/blob/7db851b66dcc6a144443ee281c5c03ae32d54253/lib/whitehall/uploader/row.rb#L136-141
Remove faulty data migration

This migration runs only on one machine in
production hence doesn't clear cache from
all nodes.
Set default expiry time for taggable helper cache

it is good to have an auto-expiration time on these
cached values otherwise they only get refreshed when
the memcached runs out of space and clears them or
the machine restarts.
Merge pull request #1803 from alphagov/set-default-expiry-time

Set default expiry time for taggable helper cache
Merge pull request #1801 from alphagov/remove-duplicate-attachments_from_columns-implementation

Remove duplicate method implementation that exists on inherited class
Merge pull request #1802 from alphagov/fix-data-migration-to-clear-cache

Remove faulty data migration
Merge pull request #1798 from alphagov/exception-watch-users-without-organisations

admin:statistics_announcements/index handles nil org
Fix ODF highlighting include ODS

ODF is the format group, ODS is the spreadsheet format, which was missing
Merge pull request #1804 from alphagov/fix-odf-extension

Fix ODF highlighting include ODS
Fix GillSans font stack

* On IE neither GillSans nor Gill Sans were matching
* In Chrome on windows, Chrome has been known to pick “Gill Sans Ultra
Bold” instead

Applying fix from admin template:
https://github.com/alphagov/govuk_admin_template/pull/41
Merge pull request #1805 from alphagov/fix-gill-sans

Fix GillSans font stack
Fix ordering of stats announcements in admin

We want the announcements returned in reverse date order, i.e. the most recent (
or future-dated ones) first.
Add support to filter announcements by date

Can ask for past or future announcements only. When asking for future statistics
announcements, they are returned in date order, i.e. the closest one first. This
makes it more likely that users will see the announcements that are most
pertinent, i.e. those that are due soon but do not yet have a linked
publication.
Wire in admin stats announcements date filtering
Highlight unlinked announcements and tweak filter

This draws attention to those announcements that have yet to be linked to a
publication.
Kill n+1 queries on admin announcements filter

We pre-fetch all the associations that get used on the index page
Filtering stats announcements that aren't linked

This enables users to view only those statistics announcements that are not
linked to a publication.
Default to showing future release announcements

It's likely that 95% of the time, a user working with statistics announcements
is going to want to see the future-dated ones, not those that have since passed.
Therefor it makes sense for the index page to default to showing the future
dated announcements for their organisation.
Warn about upcoming releases that are not linked

If there are statistics announcements due in the next four weeks that have not
been linked to a publication, then we warn the user. They can click a link to
view these announcements, making it easier to put things right.