Repository: subject cannot be loaded/edited when there are editors associated

Describe the bug
In Janeway Repository, if editors are associated with a subject, the subject page returns an error and will not load.

Janeway version
1.5.6

To Reproduce
Steps to reproduce the behavior:

1. Log in as an admin/super user and navigate to the Subjects admin page (/repository/manager/subjects/)
2. Add a new subject, select a user to serve as editor, then click Save
   
3. Click on the name of the new subject to view its page
   
4. See error: Gateway Timeout; in Sentry:

Error rendering hook press_admin_nav_block: No module named 'django.core.urlresolvers'

[edit] I thought that removing the editors in the admin panel enabled the subject to be edited, but I'm not able to access it today. Perhaps it's the huge list of potential editors that results in this issue.