bug 1462475: add db migration to cleanup after zones

[escattone]

This PR should only be merged and deployed after #4923 has been merged and deployed. It is part of the cleanup after the death of zones #4853. It adds the DB migration necessary to remove the wiki_documentzone table and the zone_subnav_local_html field from the wiki_document table.

[jwhitlock]

Goodbye, zones 💀

[jwhitlock]

If you have an existing development environment, you may want to run this migration.

(host) $ make bash
 (web) $ ./manage.py migrate

There is some leftover data after this change. A ContentType was created for DocumentZone, along with some related permissions. To clean these up, use a new command added in Django 1.11

(host) $ make bash
 (web) $ ./manage.py remove_stale_contenttypes

This is worth the Django 1.11 for me, it used to be really hard to get rid of this data.

I'm expecting another migration in the PR for bug 1455348, and then I'll start on a new sample database without zones.