refactor: reduce reliance on regex in path handling

[shreve]

This change tries to modernize the way we handle paths in nbgrader by using pathlib.Path and centralizing logic for searching and extracting information from paths into CourseDirectory.

By using Path(root).glob(pat) instead of glob.glob(root / pat), we can avoid some issues of needing to escape CourseDirectory.root because the glob will be scoped underneath a fixed path.

Introduces a new CourseDirectory.find_assignments method to help locate assignment directories across steps and students.

Fixes #1965

[github-actions[bot]]

:point_left: Launch a Binder on branch shreve/nbgrader/safer-paths