Remove the global_http_status column

- This is the third and final part of using `type` instead of `http_status`
  globally.
- I got a collation diff - I've `git add -p` only the parts of my
  `db/structure.sql` file relevant to adding this new migration.
It's probably worth pointing out that when we first tried to deploy the first chunk of work to add the `Mapping.type` column, we had to rollback on staging because of unrelated network oddness which we couldn't explain, and it was a Friday afternoon when not many ops people were around to help us investigate it. We wouldn't have been able to do that cleanly if the migration to add the new columns hadn't had `up` and `down` methods. No matter how confident we are that our code changes are good, there can always be other reasons for needing to rollback deploys.