Avoid race condition in scheduled publications after data sync

The nightly data sync from prod->staging takes a while and there's a
potential race condition whereby a document which is scheduled for
publication during this time would not get enqueued on the staging side.
That's because by the time the DataSyncComplete job runs on staging it
is now 'overdue' and so this task would not have enqueued it.

I checked the behaviour of `Sidekiq::Worker#perform_at`[1]. If you
schedule a task in the past then it will run it immediately, which seems
like a reasonable behaviour in this context.

This PR therefore removes the constraint that the `queue_missing_jobs`
rake task should only enqueue future scheduled documents. Instead it
enqueues all scheduled documents including overdue ones.

[1]
https://github.com/mperham/sidekiq/blob/a652117e2e78603450e65985e7061c133599803d/lib/sidekiq/worker.rb#L50-L51
Use Ghostscript to generate PDF thumbnails

ImageMagick cannot convert PDF to images by itself and instead delegates
to Ghostscript. By bypassing ImageMagick and going to Ghostscript
directly, we see a big improvement in thumbnail generation speed:

	Test file 1 with ImageMagick: 30.7 seconds
	            with Ghostscript:  1.5 seconds
	Test file 2 with ImageMagick: 30.7 seconds
	            with Ghostscript:  1.2 seconds
	Test file 3 with ImageMagick: 31.4 seconds
	            with Ghostscript:  1.7 seconds
Merge pull request #1645 from alphagov/faster-pdf-thumbnailing

Use Ghostscript to generate PDF thumbnails
Merge remote-tracking branch 'origin/reregister-unpublished-and-archived-documents'
Update to latest govspeak gem

Specifically, this resolves an issue with rendering multiple footnote
references.
Filter statistics announcements by organisation

The statistics announcements page in admin now defaults to showing the
current user's organisation's announcements. The filter allows other
organisations to be selected, as well as all organisations.
Include acronym in organisation select
Merge pull request #1646 from alphagov/fix-footnotes-bug

Update to latest govspeak gem
Merge pull request #1647 from alphagov/filter-announcements-by-org

Filter statistics announcements by organisation
Merge pull request #1638 from alphagov/avoid-scheduled-publish-race-condition-after-env-data-sync

Avoid race condition in scheduled publications after data sync
Merge remote-tracking branch 'origin/master'
Fixes panopticon:re_register_unpublished_editions

* Adds panopticon_api_credentials so that panopticon rake tasks can be
  run in development
* RegisterableEditionBuilder now excludes editions that do not have
  related documents