Shibboleth

I've refactored the Shibboleth authentication strategy in favour of using the standard Shibboleth Service Provider software.

This does mean we will have to run Apache + mod_shib in order to do proper authentication. However, this also means we can gain access to federations much easier. We can also start supporting built-in SAML
functionality such as WAYF and/or discovery.

When a user wants to authenticate with Shibboleth the following auth flow takes place:
1. The user browse to /api/auth/shibboleth
2. He is redirected to /Shibboleth.sso/Login?entityID=https://id.of.idp&return=https://hostname.of.tenant/api/auth/shibboleth/callback
3. This request is proxied by nginx to apache at localhost:8080 (and preserves the host header)
4. The request is passed on to mod_shib which starts the Shibboleth authentication flow and will redirect the user towards the IdP
5. When the user authenticates with the IdP, he will return to /Shibboleth.sso/SAML2/POST where the request is validated.
   This is again proxied to apache and mod_shib
6. If the request is valid, mod_shib will initiate a session and set a cookie for that user.
   Finally, the user is redirect /api/auth/shibboleth/callback.
7. This is again proxied to apache and mod_shib, where the cookie is checked against the sessionstore. If it's a valid session cookie, apache will proxy the
   request to the appserver (/api/auth/shibboleth/callback) and add the necessary headers in the request.
   These headers can be used by the OAE app server to getOrCreate a user account.
Do we need a PR for the corresponding changes against 3akai-ux? Not sure that's worth it though.