Add ondelete cascade to StudentData foreign key

[wunc]

Currently, if you want to delete a record from the students DB, you have to manually first delete any associated StudentData. We should probably add an ondelete cascade to the foreign key to avoid this. It added for ProfileData (in migration 2022_05_20_094405_add_user_on_delete_cascade.php) but not StudentData.

edit: this also needs to be added for when a user is deleted, any associated Student records or StudentData records whould be deleted.