[Assignments] Warn instructor when unlinking assignment from students who have already submitted

[milesl]

When an instructor creates a differentiated assignment that is not linked to all students/sections in a class, they receive a warning, which requires them to confirm that they did not intend to do so:

Frequently, our instructors will set a due date for the whole class, and then after the due date passes, want to create a later due date for one or more individual students.

They will inadvertently unlink the assignment from everyone else, and only linked to the new group of students, which causes all previous submissions to (temporarily) disappear from the speedgrader and gradebook.

This could be remedied by creating an additional warning message in the case that an instructor is attempting to unlink a student who has submitted the assignment:

Adding this new warning message would reduce instructor's ability to make this common error, and reduce stress for instructors who are afraid they have permanently deleted assignment submissions.

[rpeters3]

A great idea - we get many panicked calls from instructors who have gone into a quiz to give some students an extended close time and end up losing access to the majority of student quiz submissions because they've accidentally assigned the quiz new close date to just a few students and removed the quiz as being assigned to the rest of the class. This would help immensely, and as Miles has pointed out, it's exactly in line with the behavior that occurs on assignment creation - this is really just extending it to assignment edits as well.

[tstone3]

This is a great idea. Temporarily losing a full class submission just to open the submission for one or two students has happened to me in the past; most recently two days ago. Talk about a panic attack....after being on hold with canvas support hotline, I finally remembered how to get the rest of the class back. Please change this.