CSV export of student scores has incomplete header if not every student has done every exercise

Summary

When exporting student scores as CSV, the data is incomplete for the case that the first student of the export data has not worked on every assignment.

Steps to reproduce

Export student scores as CSV on current instance https://grady.informatik.uni-goettingen.de/cs1-ue/wise1920#/login/

Example Project

What is the current bug behavior?

Due to the wrong assumption that every student has completed every submission type, the header value is incorrect.

What is the expected correct behavior?

Header value of csv should be complete.

Relevant logs and/or screenshots

Possible fixes

Remove the option for csv export. Currently, the csv is generated in the frontend from the json supplied by the backend. Simply outputting this json data would be a lot simpler.
In order to properly generate a csv we would likely need to include a third-party dependency, something we should try to avoid especially for small features like this.

@hbrosen @cdamm1 @arne.bochem @abochem would this removal of the csv export option in favor of the json export be okay with you?