Course list page hangs when being accessed before the exchange is created

[enisnazif]

Expected behavior

The courses table is shown with a single available formgrader

Actual behavior

Page hangs

Steps to reproduce the behavior

Open up jupyter notebook (with all nbgrader extensions installed) prior to creation of exchange Navigate to 'Courses' tab

[enisnazif]

Opened https://github.com/jupyter/nbgrader/pull/1278 to fix this - let me know if anything else needs to be done