Serve content from rendered specialist documents

Specialist documents are now pre-rendered by the Specialist Publisher
which means a lot of logic around rendering, state and versioning has
been removed from the content API.
Specialist document format is hyphenated, not underscored

@heathd Says this is OK
Merge pull request #170 from alphagov/prerendered-specialist-documents

Serve content from rendered specialist documents

https://www.pivotaltracker.com/story/show/65352018