Add error pages to Identity

Identity doesn't currently have a webserver in its stack so we're
leakign Play messages to clients. We'll review the stack but in the
meantime this provides standard Guardian error pages on production,
tweaked so they work over https.
Add OpenGraph image to Content model
Tweak GraphicksMagick command
introducing beacon url property

The beacon url is was derived from the domain of the current page, this will not work in identity as it is on a different domain. This property will fix the beacon domain, and make it protocol relative making it usable from id pages.
Use element-assets in the picture cleaner
Merge branch 'master' of github.com:guardian/frontend into content-images-clean
removing beacon config from dev config files as they are not used
Articles which have a video element should show a main video
Update Element and PictureCleaner tests
Add video poster
Merge pull request #1748 from guardian/identity-error-pages

Add error pages to Identity
Merge pull request #1799 from guardian/content-images-clean

Article body images tidy-up and main videos for articles
Update ID Global handlers for Play 2.2
Merge pull request #1802 from guardian/identity-error-pages

Update ID Global handlers for Play 2.2
adding flag to facia metadata; adding new omniture prop
adding new clock/quote icons
adding comment icon
Merge remote-tracking branch 'origin/master' into frj-beacon-url-property
Merge pull request #1803 from guardian/fronts-omniture

Facia/fronts omniture tracking
removing rogue 'd'