Migrate nbgrader user interface assets to monorepo

[jgwerner]

There are a couple of little nbgrader UI updates used with the IllumiDesk system and need to move them into the illumidesk/illumidesk mono repo.

• [ ] Template that replaces the Formgrader label with Grader Console label.
• [ ] Add IllumiDesk image to replace Jupyter's logo image

[github-actions[bot]]

This issue has been tagged as stale due to inactivity