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.
Merge pull request #295 from alphagov/change_global_http_status_to_global_type

Change global_http_status to global_type