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.
Bullet points are not indented
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
jshint fix
add standfirst to half-card if no image
don't show headline if empty string
adding image profiles and js to load in images
Move terms and privacy policy links on Registration and Sign in pages
Merge pull request #1792 from guardian/fronts-images

Fronts images
spacing tweaks
Merge pull request #1795 from guardian/identity-registration-policy

Move terms and privacy policy links on Registration and Sign in pages
adding baseline to title min-height
Merge pull request #1797 from guardian/fronts-spacing-shizzle

Fronts spacing tweaks
Articles which have a video element should show a main video
Merge pull request #1770 from guardian/article-type

Article type style updates