Handle invalid dashboard IDs in the query string (FE)

Up until now we have not validated the selected courses, assignments and students that are represented in the URL query.

This means we will be showing "4 students" if the `student_id` query param appears 4 times, but if those are invalid values, the listbox might show a different amount of selected items.

This is even trickier to achieve with pagination in mind, as it is possible there are valid IDs in the query which are not part of the already loaded pages.