CLOUD: Add "minimal mode" in LocalWebserver

StorageWizardDialog now runs LocalWebserver in "minimal mode" for
security reasons. In this mode server uses only those handlers which
state to support it.

There are two handlers which support minimal mode: IndexPageHandler
(which handles `code` requests needed by StorageWizardDialog) and
ResourceHandler (which provides inner resources like `style.css` or
`logo.png` from `wwwroot.zip` archive).

backends/networking/sdl_net/handlers/basehandler.h

@@ -33,6 +33,7 @@ public:
 	virtual ~BaseHandler() {}
 
 	virtual void handle(Client &) = 0;
+	virtual bool minimalModeSupported() { return false; }
 };
 
 } // End of namespace Networking