Make new endpoint for most popular a/b test

This will allow us to compare most popular over-all with most popular referred from Facebook.
Remove redundant object
Respond to API errors
Remove redundant dependency
Set up new a/b test
Duplicated other most-read endpoints.
Merge branch 'ab-most-viewed-by-referrer' of github.com:guardian/frontend into ab-most-viewed-by-referrer
Merge remote-tracking branch 'origin/ab-most-viewed-by-referrer' into ab-most-viewed-by-referrer
Update Ophan API URL template
Javascript changes for new a/b test
Ignore section most popular
Add right condition for a/b test
Update condition for referrals from Facebook
Reduce time threshold for most popular
Merge remote-tracking branch 'origin/master' into ab-most-viewed-by-referrer

Conflicts:
	common/app/assets/javascripts/modules/experiments/ab.js
	common/app/conf/switches.scala
Refactor Ophan API client

Reduce multiple places where API calls are made to a single service object.
Load facebook most popular list at start-up

Because it only loads every 15 mins so need some initial data.
Merge remote-tracking branch 'origin/master' into ab-most-viewed-by-referrer
Remove regressive code that sneaked back in
If Ophan API times out give an empty list for Facebook most popular