This commit adds two new management commands for exporting and importing
mega menu content to and from JSON. These commands will enable easier
and more repeatable updating of menu content.

To export:

cfgov/manage.py export_mega_menu language output.json
OR
cfgov/manage.py export_mega_menu language (writes to stdout)

For example:

cfgov/manage.py export_mega_menu en current.json

To import:

cfgov/manage.py import_mega_menu language input.json
OR
cfgov/manage.py import_mega_menu language (reads from stdin)

For example:

cat new.json | cfgov/manage.py import_mega_menu en

The JSON format used is the content of the existing Menu object's
submenus StreamField. For testing purposes, I've created two JSON files
that can be used to switch back and forth between the current site mega
menu and an alternate one. Assuming that you have github.local aliased
to our internal GHE server, you can run these commands to toggle back
and forth.

Current menu:

curl -k https://github.local/raw/gist/chosak/4600af0d52fbe588a634ea6fee54f942/raw/5af1c1428f63ddfd7f6fc8ba9fa115b0ac17235a/current.json | cfgov/manage.py import_mega_menu en

New menu:

curl -k https://github.local/raw/gist/chosak/fdeab64f382ca0d17568cdbd717862d8/raw/9b2e8fb73062ec631c609d69c1e9ee49261b6459/new.json | cfgov/manage.py import_mega_menu en