Prepare the DOM for inline article upgrades

Also added a feature switch.
Clearer test wording
Add a list of paragraphs to the model

This is so any trail can be expanded inline.
Output the first four paragraphs of an article (rather than a standfirst) in the story package
Fix failing test
Change the language of the Trail interface to include leadingParagraphs.
Some content types are bodyless.
Fall back to regular trail style when body not available for long trail form.
Body shouldn't come up to Content, my bad.
Merge pull request #1375 from guardian/read-more

Preparation work for onwards journeys