Refactor e2xgrader-notebook configuration management and introduce environment variables

[tmetzl]

This pull request updates the configuration management for the e2xgrader-notebook Docker image to improve maintainability and support for different operational modes (base, student_exam, teacher). The main changes include restructuring configuration files, introducing mode-specific config handling, and adding cleanup steps for temporary files.

Configuration restructuring and base setup:

• Moved the nbgrader configuration to configs/base/nbgrader_config.py and expanded it with detailed environment variable support, permissions, and additional options. The Dockerfile now copies this base config for all modes. [1] [2]
• Added a new jupyter_notebook_config_override.py for base configuration, which disables the notebook banner for version 6.5.4 and is appended to the main config during build. [1] [2]

Student exam mode enhancements:

• Added mode-specific configuration: when in student_exam mode, the Dockerfile copies and installs a jupyter_backup_config.py to enable backup features based on environment variables. [1] [2]

General improvements:

• Added grouped comments to the Dockerfile for clearer separation of configuration steps by mode and purpose. [1] [2] [3]

Deprecation and removal:

• Removed the old flat configs/nbgrader_config.py in favor of the new structured base config.

These changes make the configuration more modular, easier to maintain, and allow for more flexible deployment scenarios.