Added necessay preconditions for external AD mode and hid user  template...

... menu entry in that mode
As the string is used in several places, I suggest to set it in a global variable. Also for improved usability I would link to Module Status as we are doing in other places, it can be easily done with __x(). Finally, there is a typo, we should use "Users module" instead of "module Users". This comment applies to all the references of that string.