bug 862080: Tools to define and display zone nav

* (redesign only) Populate quick links sidebar from a "Quick Links"
  section in the current document, or any parent DocumentZone documents

* (redesign only) Hide any "Quick Links" section in the current
  document, since it should get moved into the sidebar

* Helpers to deal with extracting & hiding sections in document content
  (section_extract, zone_section_extract, section_hide)

* Tweak to wiki.content.{extract,replace}Section to ignore the heading
  that defines a section

* Add DocumentZone to admin dashboard, tweak unicode repr

* Tests