add persona authentication service

I added support for persona authentication on the project. In order to authenticate in the django admin a user must be created first (by another administrator for example) and then he can decide to authenticate with persona or with username and password. This should be enough to provide authentication in the ui as well.
I also added a redirection to /admin/ from / (index). That makes sense if the ui is not plugged. If the ui is plugged, we can set a redirection (or a simple alias) to /ui/ in the apache vhost.