Human readable course titles for multi-class nbgrader on JupyterHub

[jeflem]

Course list and assignment list extensions show nbgrader's course_id (and/or the formgrader service's name) in the GUI. A course's course_id property has to be (at least) URL save if nbgrader is running in a multi-class JupyterHub setting. Thus, no white space, no specical characters (German umlauts, Chinese,...) allowed.

This PR adds a course_title property to nbgrader courses and modifies course list and assignment list extensions (server/lab/nb) to show course_title where appropriate. Using course_title for GUI (and for nothing else) allows for arbitrary course titles.

If the new course_title property is not set, then behavior is as before (show course_id). In single-class environments set c.CourseDirectory.course_title to some string. In multi-class JupyterHub environments set

c.NbGrader.course_titles = {
    'course_id_1': 'Title of Course 1',
    'course_id_2': 'Title of Course 2'
}

This PR closes #1795.

Tests pass.

Would be great if someone could carefully review this PR. It's my first contribution to nbgrader, maybe I missed something important.

[github-actions]

👈 Launch a Binder on branch jeflem/nbgrader/course_titles

[lahwaacz]

The nbgrader/nbextensions directory is gone on the main branch, so this will need to be updated for the 0.9.x version