bug 1462475: remove models/fields related to zones
bug 1462475: remove models/fields related to zones

* Delete DocumentZone model
* Remove the "zone_subnav_local_html" field from Document
  model

mozilla/kuma,SphinxKnight/kuma,a2sheppy/kuma,Elchi3/kuma,escattone/kuma,a2sheppy/kuma,escattone/kuma,Elchi3/kuma,SphinxKnight/kuma,jwhitlock/kuma,Elchi3/kuma,jwhitlock/kuma,mozilla/kuma,SphinxKnight/kuma,mozilla/kuma,Elchi3/kuma,jwhitlock/kuma,SphinxKnight/kuma,mozilla/kuma,jwhitlock/kuma,escattone/kuma,mozilla/kuma,SphinxKnight/kuma,a2sheppy/kuma,Elchi3/kuma,a2sheppy/kuma,SphinxKnight/kuma,jwhitlock/kuma,a2sheppy/kuma