Delegate authentication to plugins

Unfortunately since plugins can now override the root this is necessary
or just can't log in.

typings/pluginapi.d.ts

@@ -142,6 +142,16 @@ export { field, Level, Logger }
  */
 export const proxy: ProxyServer
 
+/**
+ * Middleware to ensure the user is authenticated. Throws if they are not.
+ */
+export function ensureAuthenticated(req: express.Request, res?: express.Response, next?: express.NextFunction): void
+
+/**
+ * Returns true if the user is authenticated.
+ */
+export function authenticated(req: express.Request): boolean
+
 /**
  * Replace variables in HTML: TO, BASE, CS_STATIC_BASE, and OPTIONS.
  */