Delete http_status columns from mappings tables
Remove (nearly) all traces of http_status

...except for in the versions helpers, which can be tidied up once
we have rewritten history so that all existing versions only refer
to type and not http_status any more.
Merge pull request #290 from alphagov/delete_http_status_columns

Delete http_status columns from mappings tables
Add a global_type column to the sites table

- Due to doing this, make global_http_status and global_type stay in
  sync with each other by setting both attributes in previous data, with
  some manual SQL in a migration. Following @jennyd's lead with what she
  did with mappings.
- This will eventually replace global_http_status, in the unresolved
  mappings process.