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
Rather than using `raw` here, how about having the presenter mark the content as `html_safe`? It's equivalent, but feels like a better place for the responsibility of deciding whether content needs to be escaped before presenting to the user.