template variables cannot be used in code macros

Steps to reproduce

1. Open the template editor
2. Insert a code macro
3. Insert a template variable into the code macro
4. Save the template
5. Create a page from the template

The user will not be prompted to insert a value for the variable.
In addition, if they attempt to edit the template, the variable will also have disappeared.

We should either allow the template variables to function inside code macros as they if they weren't inside the code macro. Or we should at least not allow template variables to be inserted into code macros.

Pre 4.3 (before we revamped page templates / template variables) template variables could be used in code macros but ONLY if the template variables was repeated somewhere else on the page outside a code macro. So even before this template variables in code macros did not work that great.