add flag to disable shutdown endpoint

[UiP9AV6Y]

the /quitquitquit endpoint allows any authenticated user to shut down the service. not sure what the point of it is, but would like to have the ability to disable this endpoint (maybe use the same approach as for the /kubecfg endpoint; use the NotImplemented handler when the endpoint is disabled)

out current workaround is to filter requests to /quitquitquit on the authenticating proxy upstream.