Manuals and sections powered by content store
Don't allow HTML in summaries

This was added because HMRC manuals did allow HTML in summaries (aka
descriptions).
Consistently use "summary" not "description"

There was an unnecessary difference between Manuals and Sections. The views use
"summary" consistently, so let's use that. Please note, however, that the data
in Content Store uses "description".
Make the presenter decide if HTML is allowed
Remove obsolete link to JSON representation

This linked to the publicly accessible Content API, but that no longer holds
the data for these pages. If there were a publicly accessible Content Store, we
could link to that.