Show warning when enabling case sharing for a user group

Refer to: http://manage.dimagi.com/default.asp?135316#766396

![image](https://cloud.githubusercontent.com/assets/3419385/4377968/e5eb814e-4353-11e4-9573-b0a906b8c149.png)
Oh, hah, yeah that is not a checkbox. Yeah this is fine.