Optionally open a particular notebook on startup

Pass a name as a parameter --notebook=name and the server will
redirect to /view/:name (urlencoded) after secure login.

This is useful for testing changes to scala-notebook, so that you can go directly from sbt to a notebook that uses the features you want to test.

Note: csrf.js would not delete the csrf cookie as expected when
loading from a non-root path, which would cause a websocket error;
this is fixed here.