fix form datetime helper

This an updated version of: https://github.com/concrete5/concrete5/pull/1392

The difference is the "$disabled" variable in this line:
https://github.com/patrickheck/concrete5/compare/fix-datetime-form-2?expand=1#diff-b6ed34e6c1137b02b89b0201c3d8a4beR109

It fixes the problem described by Andrew: https://github.com/concrete5/concrete5/pull/1392#issuecomment-34823348