Export scores for whole lecture

[meffmadd]

Is your feature request related to a problem? Please describe.
An instructor wanted to export the scores of all assignments in a single CSV file. The columns should show the assignments, and the rows show the users, i.e., a score matrix. This should be possible for both the latest and best submissions of students.

Describe the solution you'd like
Create an endpoint/change the current export endpoint to support this using pandas. Useful snippet:
final_df.merge(df, on="username", how="outer", suffixes=(False, False))