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.
added navigation-top-stories test
updated top stories test
Add new endpoint to show a/b test results
refactored section-front test
added tasks to Gruntfile
Merge remote-tracking branch 'remotes/origin/master' into update-casper-tests
Resolving conflict
Merge remote-tracking branch 'origin/master' into update-casper-tests

Conflicts:
	Gruntfile.js
Merge branch 'master' of https://github.com/guardian/frontend into update-casper-tests
Inline image horizontal spacing follows new grid
Inline images are bigger on mobile landscape
Bullet points do not perturbate vertical rhythm
Add a variant in the inline link card test
fixed indentation