Use the existing `global_http_status` database column

- Remove the migration that added `global_archive` and
  `global_redirect` columns to the sites table, following a comment
  from @jennyd.
- Exclude `MappingsController#find_global` from the before_filter of
  `MappingsController#check_global_redirect_or_archive`, otherwise
  it fails on not finding `@site`.
- Change `if` to `unless` in this new method - arguably this makes it
  less readable - I can change it if needed, but I thought this would
  avoid the else condition of things not being 301s or 410s, and the
  lines would be longer anyway due to us not using boolean columns,
  but columns that should have values or be nil.