Background
As part of T274520: Move Growth configuration to on-wiki JSON file, the Growth team is creating a system to empower communities to edit configuration of the Growth team features.
As with any other feature, this system can have security vulnerabilities. To be able to quickly reduce the damage caused by such vulnerability, we should have an off switch to disable Special:EditGrowthConfig altogether.
Task objective
Implement a config variable that would shut down the special page, and make the system use configuration from global variables, as before the change.