Change the remove_global_http_status_column migration to be reversible

- Following me making this only a `change` migration in #319, a discussion
  happened in which it was decided that it is important that it is
  reversible, otherwise things break in the event of an aborted deploy
  if the column removed does not rollback to exist again.
Merge pull request #325 from alphagov/make_global_http_status_removal_migration_reversible

Change the remove_global_http_status_column migration to be reversible