Fix taxons not appearing in admin, or being destroyed on rollback

This should work on all branches, and fixes issues I encountered with rolling back the migration.

Because the parent_id wasn't being set for the new Taxons under Tags, they didn't appear in the admin, and weren't being destroyed when this migration was rolled back.

I also ran into issues where multiple Tags taxonomies (usually created accidentally) were not being destroyed on rollback, and I couldn't rollback if somehow my Tags taxonomy had been destroyed. This patch fixes this as well. Let me know if I should split it up. Thanks!