Amendment to lesson length edit box in roll marking

In v 8.0.91 and abovetThe pencil to modify the lesson length for one week only is now stricter.
For teachers the new lesson length MUST be selected from the dropdown list of values. Other values will no longer be accepted. (Prevents teachers from entering say 35:00 for a 30:00 min lesson.) 

Administrators can still modify this list to display only values that they consider acceptable.