Implement mirror blacklisting for specific countries

[lrupp]

I have a customer who is not allowed to use mirrors from specific countries. But as there are less mirrors in his country, mirrorbrain tries to redirect him to mirrors in the forbidden countries.

While it might be an option to use the "otherCountries" field in mirrorbrain for this, I would instead love to see also a blacklist like "blockedCountries" where a mirrorbrain admin can add a list of countries where this specific mirror should not be choosen.