Change video content queries from media to element

Update the content api client from 2.0 to 2.3
Replace uses of MediaAsset and MediaEncoding with Asset
Fix indentation and prefer collect
Update tests for new ContentApi
Protect the Content class constructor

Prefer the Content companion object for instantiation.
Replace most media asset use with content-api element

Change Images trait to Elements, to include videos. Remove assumptions and rules for finding images in media assets.
Add Element class to frontend's model.
Use Content object instead of constructor
Add tests for element model

Change Element class inheritance structure to use traits.
Update Elements trait to support test requirements.
Include body video assets in videos list.
Merge branch 'master' of github.com:guardian/frontend into remove-media-queries

Conflicts:
	applications/app/views/fragments/galleryBody.scala.html
	article/app/controllers/ArticleController.scala
	common/app/common/editions/Au.scala
	common/app/contentapi/ContentApiClient.scala
	common/app/model/content.scala
	common/app/model/meta.scala
	common/app/model/trails.scala
	common/app/views/support/package.scala
	core-navigation/app/controllers/MoreStoriesController.scala
	core-navigation/app/feed/MostPopularAgent.scala
	core-navigation/test/MoreStoriesControllerTest.scala
	football/app/model/LiveBlogAgent.scala
	front/app/controllers/front/Front.scala
	project/Frontend.scala
reorganise casper tests into folders based on projects
added discussion folder
Merge remote-tracking branch 'origin/master' into reorg-casperjs-structure
Update model for content client 2.6, include video duration
Merge branch 'master' of github.com:guardian/frontend into remove-media-queries
added core-navigation grunt task
Merge branch 'master' of github.com:guardian/frontend into remove-media-queries
Content API timeout value is now configurable

It now uses `content.api.timeout.millis` in the configuration (which was already there) via the Configuration object.

This is mainly useful for OSX developers who like to raise their timeout value to 5000ms.
Use thumbnails and video posters as main pictures when needed
Update tests with common model changes
Live summary truncation gradient looks good in all browsers Fixes #1597
Merge pull request #1583 from guardian/reorg-casperjs-structure

Reorg casperjs structure