An option to lock teacher marks on a checklist so that they can only be changed by a 'supervisor', once they have been set.

1. Set checklist to 'Teacher only' or 'Student and Teacher'
2. Set 'lock teacher mark' to 'yes' (will need some help documentation)
3. Once the teacher has set a mark to 'yes' or 'no', disable the dropdown box and ignore any attempts to change the mark
4. Allow someone with the capability 'mod/checklist:updatelocked' (ie a 'supervisor') to still change the marks

(Add the 'lock teacher mark' setting to backup/restore