Document manual overrides to teacher survey info

A number of errors emerge in teacher data recorded on the Staff page of the workbook. It's most expeditious to modify these on the TeacherSurvey table, without uploading again.

However, we shoould be able to record that such as manual override has been done, to explain the discrepancy between the workbook data and the recorded result.

Add new fields to TeacherSurvey to accomodate this.