refactor: Read AFV URLs from bootstrap.json

This also removes the possibility to change the URL on the fly via the
UI.
Fixes #257

src/swiftguistandard/swiftguistd.cpp

@@ -671,7 +671,7 @@ bool SwiftGuiStd::startAFVMap()
     //! \todo KB 2019-11 AFV map workaround
     if (sGui && !sGui->isShuttingDown())
     {
-        sGui->openUrl("https://afv-map.vatsim.net/");
+        sGui->openUrl(sGui->getGlobalSetup().getAfvMapUrl());
     }
 
     return true;