Refactor/Cleanup NetworkRequest and Related Code (#4633)

Cleanup unused code (Twitch v5)
Add json methods that simplify sending JSON. This also sets the Accepts header, since here, when JSON is sent, only JSON is accepted as a response (this is only done in Helix).
Clarify helix request creations
Cleaned some clang-tidy suggestions in Network{Request,Private}

src/common/NetworkPrivate.hpp

@@ -68,6 +68,6 @@ private:
     QString hash_;
 };
 
-void load(const std::shared_ptr<NetworkData> &data);
+void load(std::shared_ptr<NetworkData> &&data);
 
 }  // namespace chatterino