refactor: more net cleanup

This runs clang-tidy on some other files in launcher/net/.

This also makes use of some JSON wrappers in HttpMetaCache, instead of
using the Qt stuff directly.

Lastly, this removes useless null checks (crashes don't occur because of
this, but because of concurrent usage / free of the QByteArray pointer),
and fix a fixme in Download.h

launcher/net/Mode.h

@@ -1,10 +1,5 @@
 #pragma once
 
-namespace Net
-{
-enum class Mode
-{
-    Offline,
-    Online
-};
+namespace Net {
+enum class Mode { Offline, Online };
 }