text: Title: Option to create code task cells should be disabled
  Body: > Regarding the drop down menu, yes, I think it would be better to only have the "task cell" option visible for markdown cells.
  
  [The documentation](https://nbgrader.readthedocs.io/en/stable/user_guide/creating_and_grading_assignments.html#manually-graded-task-cells) asserts this too "If you select the “Manually graded task” option (available for markdown cells), the nbgrader extension will ..."  However, the "Manually graded task" option seems to be available for code cells as well:
  
  ![image](https://user-images.githubusercontent.com/1276278/66357669-a2be8480-e924-11e9-9908-b3bfe077d381.png)
  
  I don't really care either way.  It's just that I'm implementing a hopefully 100% compatible version of nbgrader from scratch for https://cocalc.com, and I need to know what to implement...
  
  _Originally posted by @williamstein in https://github.com/jupyter/nbgrader/pull/984#issuecomment-539255861_
  
  cc @danielmaitre --- would disabling this option break your use case?