Shibboleth authentication

This PR replaces the SAML 2 authenticator with Shibboleth.

It has been tested with the following identity providers:
 * [TestShib Two](https://www.testshib.org)
 * [University of Cambridge IdP](https://shib.raven.cam.ac.uk)

It currently relies on a [custom branch](https://github.com/simong/passport-saml/tree/shibboleth) of the passport-saml library for which there is a [PR pending](https://github.com/bergie/passport-saml/pull/11).

Documentation for setting it up is available on [Confluence](https://confluence.sakaiproject.org/display/3AK/Authentication#Authentication-Shibbolethauthentication).