Implement NetClient PushGuiMessage using parameter pack and ScriptInterface::CreateObject from D2080 / rP22528 to replace remaining Eval function calls.

Specialize ToJSVal<char[n]> to allow passing string literals as arguments.
Freeze NetClient GUI messages.
Supersede related c-style casts with static casts.
Remove few unneeded conversions.

Differential Revision: https://code.wildfiregames.com/D2267
Tested: clang 8.0.1, Jenkins