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.
removing beacon config from dev config files as they are not used
Merge remote-tracking branch 'origin/master' into frj-beacon-url-property
Merge pull request #1790 from guardian/frj-beacon-url-property

introducing beacon url property