Define Util.removeChildren function

Currently, the code in Tutor object removes all children of a
DOM node with code like this:

    while (node.firstChild) {
        node.removeChild(node.firstChild)
    }

Put this code in a function called Util.removeChildren and call
this function from Tutor object where all children of a DOM node
need to be removed.
Do not display 'Next lesson' link for last subunit

When a user completes the last subunit, i.e. Unit 21.3,
satisfactorily, the tutor displays 'Next lesson' as the advice.
This is a bug because there is no next lesson when the user is
at the last subunit. Therefore, fix this bug so that the 'Next
lesson' advice is not displayed when the user completes the last
subunit satisfactorily.