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
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
Stub a whitehall admin front-end style guide

* Tricky naming for this, as “style guide” and “front-end” are both
already overloaded terms
* Page is intended to have the same purpose as /style-guide in other
admin apps, which documents design patterns that differ from Bootstrap
Add grid example to whitehall style guide

* Include Bootstrap 2.0.3 caveats
Add form example to style guide

* Indicates how default legend and fieldset styles create a layout that
separates headings from content
Shim govuk_admin_template helpers into admin

* Rather than porting Whitehall over to the template, which would take
a long time, begin adapting some of the concepts the template introduces
* Include CSS helper classes for admin UI
* Include theme variables used within the template
Emulate form styles used in govuk_admin_template

* Use a class, form-large, to avoid applying this everywhere
* Reset many of the styles applied by Bootstrap 2
* Default to a larger font-size
* Document styles in the style guide
Apply form-large styles to new stats announcement

* Add form-large, form-group wrappers and form-control classes to form
elements
* Apply checkbox and radio styles directly to labels where appropriate
— form builders make it harder to change the markup
Create form helper for both save/cancel as buttons

* Make the cancel action clearer by using a grey button style, but
separate it from the main submit action to prevent accidental clicks
* Create gutter classes and mixins to specify a large gap between
elements, in this case two buttons
Cleanup new stats, edit and change date templates

* Move or (remove where appropriate) the warning message to render grid
correctly
* Remove warning message when performing an edit action, presume that
at this point the user understands the consequences of these actions
* Create styles for item and actions within a header, this ought to be
extracted into a generic pattern at a later date
Update button wording to indicate publish

* When a stats announcement is created, edited or has its date changed,
the changes are immediately published — the button text needs to make
this clear
Apply form-large styles to date change form
Cleanup stats announcements list

* Put header at the top of the page
* Move warning down and give better vertical spacing to create
announcement action
* Rename search to filter to match other filter forms on Whitehall
* Make the search button a Bootstrap styled button