GH-3234 Add support for custom meta URLs at build time

This is not particularly interesting for non-developers.
Also includes some internal restructuring of URL constants in general.

api/logic/minecraft/auth/YggdrasilTask.cpp

@@ -25,7 +25,7 @@
 
 #include <Env.h>
 
-#include <net/URLConstants.h>
+#include <BuildConfig.h>
 
 #include <QDebug>
 
@@ -42,7 +42,7 @@ void YggdrasilTask::executeTask()
     // Get the content of the request we're going to send to the server.
     QJsonDocument doc(getRequestContent());
 
-    QUrl reqUrl(URLConstants::AUTH_BASE + getEndpoint());
+    QUrl reqUrl(BuildConfig.AUTH_BASE + getEndpoint());
     QNetworkRequest netRequest(reqUrl);
     netRequest.setHeader(QNetworkRequest::ContentTypeHeader, "application/json");