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.
If we're bothered, we can put it back. It's worth remembering, so might be worth it as an exercise to help that memory. But are we going to use it in practice? Naaaaah :zap:  :gun: