Data migration to change various slugs

There's a corresponding puppet change which adds redirects from the old
to new slugs. It would be good to deploy the puppet changes shortly
after this data migration is run.

https://github.gds/gds/puppet/pull/664

Relevant tickets:

https://www.pivotaltracker.com/story/show/48656295
https://www.pivotaltracker.com/story/show/49053747
https://www.pivotaltracker.com/story/show/47807097
https://www.pivotaltracker.com/story/show/49187057
https://www.pivotaltracker.com/story/show/42172445