Add OpenGraph image to Content model
Tweak GraphicksMagick command
Use element-assets in the picture cleaner
Merge branch 'master' of github.com:guardian/frontend into content-images-clean
Articles which have a video element should show a main video
Update Element and PictureCleaner tests
Add video poster
Merge pull request #1799 from guardian/content-images-clean

Article body images tidy-up and main videos for articles