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.
Rails generated an up/down migration for me by default, but I decided to use change as I remember someone saying to me something about a lot of migrations don't need to be reversible. I might be misremembering, but anyway, we're removing this column forever, right? :-\