snapshot
<meta tags properly terminated with />  (conversion to XML for testing)
</h3> closing tag moved to the proper position--outside the <form></form> element  (conversion to XML for testing)
All work on this branch will be terminated with this checkin.

This feature is incomplete in its implementation:
* The visibility icons are shown correctly and respond to mouse clicks by doing the proper things.
* The unit tests for this new code are not yet complete; a scheme for performing such tests has been worked out. See 'test_unit_visibility.py'.
* The code has not undergone a thorough scrubbing to remove superfluous comments and cleanup formatting issues.
* There is some controversy about the page-refresh technique for updating the icons on the page when the user requests a change to unit status. This code will simply call for a full page refresh, but there are probably more elegant ways of handling the update.
* While this feature was intended to pulled into the edX/OpenEdx master branch when ready for prime time, preparation of an official Pull Request has not been done.