Code review comment for lp:~jpds/launchpad/fix_361650

= Summary =

Launchpad should know which mirrors are set as official country mirrors (eg. gb.archive.ubuntu.com), so that we can track these easily on mirror listings.

This branch also includes a database change (patch-2207-20-0.sql) which adds a official_country_mirror column to distributionmirror. It also includes two constraints to ensure that there can not be more than one (archive|releases) mirror per country.

New mirrors have been added to current(-dev).sql for testing purposes too.

A mirror should only be eligible for country mirror status when:

1) it has been reviewed and set as an official mirror by an admin,
2) it has been probed for integrity,
3) it has an HTTP URL set.
4) there is no country mirror for that mirror type in the country.

Checks have been added to the code to ensure that these conditions are met.