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
We can now use absolute URLs around the site.

 We need to support different domains (e.g. for Identity).

 We no longer need most of the complexity of LinkedTo (it has not disappeared, it has just moved to Fastly following the domain switch).

If you have the site.url property set e.g. sit.url=http://www.theguardian.com than lniks built by LinkTo will be absolute. If not they will be relative to the root of the site.

By default dev machines will use relative urls.
Small fixes to LinkTo on back of broken tests
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