Use nickname in folder structure

When downloading all submitted files from a task, a folder structure is created with one folder per user. The name of the user folder contains the first and last name of the user and additionally either the user name (nickname) or a number automatically generated by OpenOlat like u3685010213. This is confusing when both variants appear together in the folder structure.

For standardization, only the user name should be used as the last part of the file name: firstname_lastname_username