Teacher mode / student mode

We need some kind of student mode where the timestamp editor button is hidden. There are two ways to implement this:

• A global mode
• Per notebook lock / unlock

From discussing with Alan, I think we should go for the notebook lock option. There should be a button to "lock" the notebook, so that the timestamps cannot be changed anymore. There should also be a smaller button or a menu option to resume editing. Note that locking the timestamps is not a security feature, since the source can be edited anyway, but a convenience feature so users of the notebook are not distracted.