Review use cases for user's username field

[tcompa]

Let's collect here any relevant use case for a non-optional username.

• Showing the info about who owns a given task - ref Display ownership info in task-group "Info" fractal-web#600
• Possibly useful in creating default dataset zarr_dir? Actually no, we went for a different route based on user.project_dir (ref ref Set a default for "Zarr dir" #1934).

[tcompa]

Conclusion: Let's not remove it for now, but also not do anything more until we know the longer-term scope

(fractal-analytics-platform/fractal-web#672)