Default news image on orgs

New Image Uploader
Associations on Organisation
Admin form to upload the image
use the default news image from the first lead org if present
Fix tests for the lead image and new article presenters
Remove test and add one in organisation test
Only show the image on the admin page if set
Add lead images to world location news articles

Added default news article image to worldwide organisations

Fix LeadImagePresenterHelperTest

Fix world_location_news_article_presenter
Test for worldwide_organisation default news image
Add test for WorldLocationNewsArticlePresenterTest
Move the logic for lead_image_path into the helper
Move default image_fields into it's own partial
Fix lead_image_presenter_helper for placeholder

Also rename method to be placeholder_image_exisits?
include find_asset in case_study_presenter
Merge pull request #426 from alphagov/default-news-image

Default news images for news articles

https://www.pivotaltracker.com/story/show/46531703