Option to disable adding new files to legacy course files

Enable legacyfilesinnewcourses, and make sure legacyfilesaddallowed is checked. Create a course, and set Legacy Course Files to on in the course settings. In the course admin block, follow the link to Legacy Course Files. Edit the files, and verify that you can add files and directories. Save the files you have added. Set legacyfilesaddallowed to unchecked. Return to the legacy course files editing page. Verify that you don't see the 'Add...' and 'Create folder' buttons, and that drag and drop upload gives you an error. Verify that you can delete files and folders.