assignment filter on requests page does not allow selecting multiple values

When using the assignment filter on the requests page, you can only select one value. While this is good behavior for most of the filters, for the assignments you want to be able to select multiple courses. For instance, you would want to receive requests for both assignments 1 and 2, but not for 3.

I put medium priority because this is a very common use case for the TAs at the shared question session.