Bump to latest gds-sso

Newest version sets an appropriate response content-type, which will
stop slimmer from attempting to process the empty responses on reauth
calls
Override gds-sso authentication failure template

This page is rarely seen, but because it lacks the expected wrapper
div in the gds-sso gem, slimmer throws a wobbly when it tries to process
it. By overriding the template here in whitehall, we can ensure the
appropriate markup is present to keep slimmer happy.
Merge pull request #1505 from alphagov/bump-gds-sso

Fix gds-sso/slimmer-related issues
Consistent feature new item form
Simplify header CSS by using govuk_template defaults

Lots of previously-specifically-whitehall CSS is now just the default, as doesn't need overrided here.

* Remove custom whitehall bar and use the standard gov.uk bar
* Remove custom whitehall colours, overrides. These have been migrated to
* Specify border-bottom, as that's usually on wrapper, which whitehall doesn't have
* Remove custom dept styling CSS and class hooks, which wasn't actually be applied anywhere
Convert some rogue tabs to spaces.
Merge pull request #1499 from alphagov/add-explanation-featuring-form

Consistent feature new item form
Merge pull request #1507 from alphagov/remove-tabs

Convert some rogue tabs to spaces
Merge pull request #1506 from alphagov/simplify-header-css

Simplify header CSS by using govuk_template defaults
Use polymorphic URL for classification delete link.
PubDocRoutesHelper is always explicit about locale

:polymorphic_url, used by :document_url in PublicDocumentRoutesHelper,
uses the current locale when generating routes if a locale is not
explicitly given.  This meant that when in a foreign locale it would
generate a url in that locale when given an english edition, even if
that edition wasn't translated into the locale.

This commit changes the behaviour to always be explicit about locale
for translatable edition types with the following logic:

• If a :locale option is given, use that.

• Otherwise if we're in a foreign locale and the given edition is
  available in that locale, use that locale.

• Otherwise, use english.
Don't use LocalisedModel in front-end tests
Don't use public_document_path in cucumber steps to test locale
Fix test failing on CI
Merge pull request #1496 from alphagov/fix_document_url_helper

PubDocRoutesHelper is always explicit about locale
Merge pull request #1508 from alphagov/topical_events_delete_button

Use polymorphic URL for classification delete link.

https://www.agileplannerapp.com/boards/173808/cards/3352
Offsite links
New UI for featuring and unfeaturing items for topics, topical events, organisations and world locations.
Added the ability to feature offsite links on topics, topical events, organisations and world locations

https://www.pivotaltracker.com/story/show/66668192
https://www.pivotaltracker.com/story/show/70537384
https://www.pivotaltracker.com/story/show/70741400
https://www.pivotaltracker.com/story/show/70537546
https://www.pivotaltracker.com/story/show/70537432
Use pdf-reader gem to count pdf pages.

Further to https://github.com/alphagov/whitehall/pull/1458

Although pdfinfo is slower than imagemagic in some cases, imagemagic is
significantly slower for pdfs with a large number of pages:

```
$ time pdfinfo test.pdf
Title:          Salisbury Plain army basing programme OEA
Keywords:
CreationDate:   Tue May 20 12:46:01 2014
ModDate:        Tue May 20 12:46:01 2014
Tagged:         yes
Pages:          836
Encrypted:      yes (print:yes copy:yes change:no addNotes:no)
Page size:      595 x 842 pts (A4)
File size:      7161312 bytes
Optimized:      yes
PDF version:    1.6

real  0m0.012s
user  0m0.004s
sys 0m0.007s

$ time identify -format %n test.pdf
836

real  1m23.043s
user  1m21.184s
sys 0m1.810s
```

The pdf-reader gem is near instant for both this example, and the one
used in the previous PR.
Nesting and grammar
Merge pull request #1504 from alphagov/offsite-featurings-orgs

Offsite links