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

This is mostly the same as [the previous attempt](https://github.com/mozilla/kuma/pull/2047) that was reverted as part of yesterday's performance madness. Two main differences:

* `get_body_html` now does section ID injection and link annotation, two things that were missing before.

* The `get_*` model methods are now called in the view, rather than from the template. That should make it clearer what's going on with these fields, closer to code that mucks with content.