AB test to see if underlined links are clicked more frequently
Fix returned reference
Underline test off by default
Add two new variants

- Variant #3 makes inline links high visibility.
- Variant #4 makes inline links hard to see.
Scope tests to article body (to avoid byline hightlights)
Merge branch 'master' of https://github.com/guardian/frontend into underlined-links

Conflicts:
	common/app/assets/javascripts/modules/experiments/ab.js
	common/app/conf/switches.scala
Merge pull request #2647 from guardian/underlined-links

AB test to see if underlined links are clicked more frequently than not underlined ones.