Improve Exports

This MR improves and normalizes all exports throughout the project.

• All CSV files now contain the same information as the XLSX files
• Added additional information to many exports
• Added custom export suitable for grading at the course edition
• Used human attribute names / model names where possible (to improve consistency throughout the application)
• Fixed &quot appearing in CSV exports

Custom export for grading

Grades can be entered per group on the projects tab, and will be automatically shown next to students. Only groups that are approved and students belonging to those groups are in the CSV.

There is support for more than two coaches in the additional coaches fields.

Closes #42 (closed)