Added a 'DogpileAction'.

This is designed to soak up large numbers of hits on a single page (e.g. end of live football match, election of Pope). If the application is currently fetching and rendering that page, then this action will await that result instead of duplicating the effort.
Tweaking cache key a bit
Moved Facia to use the DogpileAction.

While in there also shifted some of the boilerplate into its own file to get it out of the way of the working code.
Linked to docs for concurrency level.

Hopefully people read it before changing it
Merge branch 'master' of github.com:guardian/frontend into dogpile

Conflicts:
	common/app/conf/switches.scala
Merge branch 'master' of github.com:guardian/frontend into dogpile

Conflicts:
	common/app/conf/switches.scala
Merge pull request #2983 from guardian/dogpile

Added a 'DogpileAction'.