Sanitize some values

The current implementation of contribook is vulnerable against stored XSS. This allows basically any unauthenticated user to inject arbitrary JS into owncloud.org

I have not tested this as I don't have a local running Contribook instance. It would be helpful if I could be granted SSH access to staging.owncloud.org so that I can perform such tests without further hassle ;-)

@karlitschek Please review.

@tomneedham This needs to get deployed on the new oC.org website. THX!