Restrict login

Correct me if I'm wrong: 
Right now this can only really be deployed behind a firewall or some additional auth, right? Even when repos are restricted to particular organizations or users, _anyone_ with a Github account can log into your Strano installation and use it to fire stuff off using _your_ ssh key.

This commit adds a new configuration option, `open_login`, which restricts users' ability to access the app at all, based on the user's github login or organization membership. This makes it much safer to deploy this excellent app on a publicly accessible server.

Also turned on `disallow /` in robots.txt, for philosophically similar reasons.