Add common options to ExternalFileAssignment
Have radio buttons at the top that say:
All questions for each student are in the same file - if this is selected, then we don't need the question list.
All data types are in the same file - if this is selected, then we don't need the data type list
All students and questions are in the same file. This should be the default option if there is only one external file selected. If chosen, then all
students and questions are selected by default (or the lists aren't shown).
Do we ask them which types of data they have external files for, or do we just let them select them from the list of data they've suggested? I don't know ...