Prevent hyperlink handler for potential dangerous URIs

This prevents the user from clicking on URIs starting with `javascript:` or `data:`. The reason behind this is that this may be used to trick users in executing dangerous JS when viewing an untrusted document. (which is the case in our deployment for ownCloud)

I'm not absolutely happy with that patch since it uses a blacklisting instead a whitelisting approach, but I consider it a feasible approach. Especially, considering all the possible values. (`mailto:foo@bar.com`, `ftp://`, `skype://`, etc...)