Change data source to the Content Store

This change:
* introduces the Content Store as the single source of data
* changes section rendering to allow both a body and subsections to be displayed (needed for HMRC manuals)
* cleans up the code for rendering Content API and static content
* cleans up govspeak-related code (since content in the Content Store is pre-rendered already)

What's **not** done:
* removal of the employment income manual JSON
I think your point is valid, but it's probably best to do a separate change to fix that.