Add images to documents featured within organisations

* Introduced a new form to edit the EditionOrganisation object.  We
  previously used an inline form to POST to the update action to either
  feature or unfeature an EditionOrganisation, but we now need to be
  able to upload an image too.

* Added a new EditionOrganisationImageData model with a new
  EditionOrganisationImageUploader.  The model and uploader are
  essentially copies of ImageData and EditionImageUploader.  The
  motivation for introducing these new objects, rather than reusing
  those that they were copied from, is because we might want to do some
  different processing on images uploaded as part of featuring a
  document. *NOTE* I've named the association `#image` on the
  EditionOrganisation as `Edition#edition_organisation_image_data` felt
  pretty cumbersome.

* The EditionOrganisation validates the presence of the image only if
  the edition is being featured in that organisation.  This relies on
  the EditionOrganisation#featured? method provided by ActiveRecord.

* Updated the hero_rotator partial to deal with displaying featured
  images for organisations and the Edition#lead_image for countries.  We
  should be able to simplify this if we bring featuring editions for
  countries into line with that of organisations. *NOTE* Ignoring
  whether or not we bring featuring editions within countries into line
  with doing the same for orgs, we should still be able to tidy this
  partial up a little.

* Updated the EditionOrganisationControllerTest to login in the setup,
  as all tests required an authenticated user.

* Updated the OrganisationsControllerTests (public facing and admin) to
  add an image to featured edition_organisations where necessary.

* Removed the test from OrganisationsControllerTest that was checking
  that we displayed a generic image if no lead image was available for
  the Edition.  This is no longer necessary as you must always select an
  image when featuring the edition.

* Split the features in administering-organisations.feature so that
  featuring documents is tested separately to ordering the featured
  documents.
Extract duplication into a featured_edition_organisation factory.
Require & use alt text for image for a doc featured within an org.

Note that the logic in the `hero_rotator` partial template has become
even more horrendous, but we should be able to simplify it later as
explained by @chrisroos in d70ac6266753464c7bd11fa5c0de6164f53d9cda.
Ensure all edition_organisations are valid.

In d70ac6266753464c7bd11fa5c0de6164f53d9cda, validation was added so
that a featured edition_organisation has to have an associated image.
Since none of the existing edition_organisations will have an associated
image, I've elected to mark all of them as unfeatured. This will mean
that they will all need to be marked as featured again, but this time an
image and alt_text will be required.

I did consider copying over data from NewsArticle#lead_image, but it's
been possible to feature any document type for a while (not just
new articles which are the only ones to have a `lead_image`), so I
thought it was simplest just to unfeature them all.
New draft of doc featured within org should retain image & alt text.
Remove feature image & alt text when unfeaturing doc within org.

This starts making me think it might be better to have a separate
Featuring model, but this should work for now.
Cache uploaded image in case of validation failure for doc org featuring.
Ensure that edition_organisation always has edition & organisation.
Attempt to make edition_organisation form look presentable.

Also add cancel link back to admin organisation page.
Show the featured documents after featuring one.

I've added some javascript which will focus a tab based on the
anchor part of the URL. This should actually work on all the admin
bootstrap tab pages.
Any kind of document can be featured, not just news.
Hopefully more intuitive button text.

I found it a bit unintuitive that you click a button with the text
'Featured' to unfeature an edition. I think this is clearer: the
button text now only ever describes what clicking the button will
do.
Make the form a bit less wide.
Make the form slightly friendlier to use.