Merge pull request #931 from minrk/readonly The notebook now supports a `--read-only` flag, which allows users to view all notebooks being served but not to edit them or execute any code. These actions are not allowed and the buttons, shortcuts, etc. are removed, but the requests will raise authentication errors if they manage to send the events anyway. Save/print functions remain available. This flag can be used in two modes: 1. When running an unauthenticated server, one can run a *second* read-only server in the same directory on a public IP address. This will let users connect to the read-only view without having to worry about configuring passwords and certificates for the execution server. 2. When running a server configured with authentication (and hopefully an SSL certificate), starting it with `--read-only` allows unauthenticated users read-only access to notebooks. This means that the same server on a single port can be both used by authenticated users for execution and by the public for viewing the available notebooks.