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.
Make new endpoint for most popular a/b test

This will allow us to compare most popular over-all with most popular referred from Facebook.
Remove redundant object
Respond to API errors
Remove redundant dependency
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
Set up new a/b test
Update model for content client 2.6, include video duration
Merge branch 'master' of github.com:guardian/frontend into remove-media-queries
Merge branch 'master' of github.com:guardian/frontend into remove-media-queries
Duplicated other most-read endpoints.
Merge branch 'ab-most-viewed-by-referrer' of github.com:guardian/frontend into ab-most-viewed-by-referrer
Merge remote-tracking branch 'origin/ab-most-viewed-by-referrer' into ab-most-viewed-by-referrer
Update Ophan API URL template