Change gradereport/overview:view default setting to 'Not set' for the role of teacher and non-editing teacher in new installs

Currently the default setting for gradereport/overview:view is 'Allow' for the roles of teacher and non-editing teacher. This doesn't make much sense, as the overview report simply lists all the courses a student is enrolled in with the total grade for each course, and teachers obtain the message "There are no courses yet".

Thus, for new installs it would be better if gradereport/overview:view is not set, and documentation is provided for administrators of existing installations explaining how to simplify the gradebook for teachers by removing the overview report link.