Issue #1964: DB fields for generated content - quick links, zone nav, and body HTML

This moves content extraction for quick links, zone nav, and the doc body HTML into the `render()` method. At document view time, this content is pulled from DB fields instead of being re-extracted with every view.

The only exception is when the DB fields are null, which will be the case when / if this PR is initially deployed. Then, the content *will* be extracted on demand, at least until the next time documents are re-rendered or edited. 

That means performance gains will ultimately depend on performing a full-site re-render.