* Add a migration to drop the old countries field.
* Update all templates/views/utility methods to point at the new country
  field and dereference it as necessary.
* Add the flags images to a few views where it makes sense.
* Cleanup the download page layout quite a bit.
* Bump the mirror status JSON version to 3; add country_code attribute.

Signed-off-by: Dan McGee <dan@archlinux.org>