NOISSUE Use constants for user agents

Here lies yet another early-stage move to debrand the MultiMC codebase,
as well as reducing the burden of updating strings across the codebase
for a future MultiMC6.

api/logic/net/Download.cpp

@@ -15,6 +15,7 @@
 
 #include "Download.h"
 
+#include "BuildConfig.h"
 #include <QFileInfo>
 #include <QDateTime>
 #include <QDebug>
@@ -94,7 +95,7 @@ void Download::start()
             return;
     }
 
-    request.setHeader(QNetworkRequest::UserAgentHeader, "MultiMC/5.0");
+    request.setHeader(QNetworkRequest::UserAgentHeader, BuildConfig.USER_AGENT);
 
     QNetworkReply *rep =  ENV.qnam().get(request);