text: Title: "manual grading" screen unable to format any math containing a prime symbol `'`
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  ### Operating system
  MacOS 10.14.4 (18E226)
  
  ### `nbgrader --version`
  0.6.1
  
  ### `jupyter notebook --version` 
  6.0.3
  
  ### Expected behavior
  In one of the courses I am teaching, we wanted to use nbgrader to automate some of the grading. Until now it's working beautifully -- thanks for this.
  For this week, we have a notebook containing function derivatives, like so:
  $$
    y'(x) = f(x, y(x))
  $$
  which sets correctly in my `source` and `release` versions, looking a little like this:
  ![image](https://user-images.githubusercontent.com/1742490/81955939-028fb300-960b-11ea-9a8c-5f12efb74daa.png)
  
  ### Actual behaviour
  If I then open a student submission, at first I am able to see the math, like so,
  ![image](https://user-images.githubusercontent.com/1742490/81956143-3ff44080-960b-11ea-97e2-71cb43952014.png)
  
  but when it is about to replace the text with a MathJax formula, it looks like so:
  ![image](https://user-images.githubusercontent.com/1742490/81956122-3965c900-960b-11ea-8f5e-ba6f2efd7c6f.png)
  
  When this happens, I get these errors in my terminal:
  ```
  [W 17:49:18.142 NotebookApp] 404 GET /static/components/MathJax/jax/output/HTML-CSS/fonts/STIX/fontdata.js?V=2.7.7 (::1) 2.53ms referer=http://localhost:8889/formgrader/submissions/ccc77ef030ea4ae5bda1d7d0e787089c/?index=0
  [W 17:49:20.909 NotebookApp] 404 GET /static/components/MathJax/fonts/HTML-CSS/TeX/otf/MathJax_Script-Regular.otf (::1) 1.49ms referer=http://localhost:8889/formgrader/submissions/ccc77ef030ea4ae5bda1d7d0e787089c/?index=0
  ```
  This only happens in the formulas containing a prime/derivative sign, weirdly enough.
  
  ### Steps to reproduce the behavior
  I get the same results using a different browser and also using a different machine (MacOS 10.14.6, jupyter 6.0.2, nbgrader 0.6.1)
  
  I am not sure how I can add a MWE but if it seems useful later I can try making a test assignment and a submission, or I could share an anonymized submission from the real assignment. Please let me know which one is preferable.
  