text: Title: Ability to show/toggle all student names in formgrader
  Body: Follow up on:
  https://github.com/jupyter/nbgrader/issues/575 
  ... which led to:
  https://github.com/jupyter/nbgrader/pull/617
  
  I would highly appreciate the possibility to **toggle the visibility of all student names--with one click**. We do grading anonymously and I think it's a good default.
  
  However, when students want to discuss individual issues **after they've received their grades/feedback**, I currently have to toggle up to 200 submission names to find the formgrader version of their submission. 
  Thanks for any suggestions! 
  
  