Changes admin config countries to display only active countries

I didn't like to have all of the countries in a paginated list, it was slow to find the country I wanted, so I came up with this approach:
The countries page will show up on the list only the active countries - with link to deactivate them - and there is a select_tag with all of the countries where you can just select a country and it'll be activated.
It is much faster that way =)