Know within which country a bridge is located

This has many uses, but in particular this will allow us to do legacy/trac#12843 (moved). I suggest we not bother rewriting large portions of the code to satisfy this and simply add another attribute to the Bridge class specifying its country. As soon as we have this information we can do more smart things with it (and those can be other smarter tickets).

For simplicity, we can only rely on the the ORPort netblock. This is what the BA checks. In the future, when ORPorts are disableable, we'll need to rely on the addresses in the transport lines, but that can be dealt with later, I think.