Fix logout triggering 404 error

See https://talk.tiddlywiki.org/t/logout-error-xmlhttprequest-error-code-404/5590/5 for details

We can't POST to the logout endpoint without triggering authentication, so we report in advance whether logout is supported.

core/modules/server/routes/get-status.js

@@ -21,6 +21,7 @@ exports.handler = function(request,response,state) {
 		username: state.authenticatedUsername || state.server.get("anon-username") || "",
 		anonymous: !state.authenticatedUsername,
 		read_only: !state.server.isAuthorized("writers",state.authenticatedUsername),
+		logout_is_available: false,
 		space: {
 			recipe: "default"
 		},