"Collecting All Submissions" continues to be a problem

[reidka]

1. We still see examples of instructors inadvertently collecting after marking has been done Change UI for "Collect all submissions" #1705
   Part of this problem is that the Collect All Submissions link looks the same all the time. The alert box appears all the time, but is easy to ignore.
2. It isn't obvious to new users what Collect All Submissions does. We need some documentation to explain this and put it in a place that users will see.
3. It would be good to implement an undo collection feature, but where to put this button is challenging because it gives instructors another way to destroy data, so they only want to use it when they have mistakenly collected. How do we indicate that this is a dangerous operation without obscuring it so much that no one can find it? (We could make it easy to revert to a previous collection when the current collection is empty - no marks or annotations.)
4. It isn't obvious when collection is finished. Download all submissions only downloads some of the submissions #2359
5. It would be nice to add the ability to collect a few submissions using the checkbox column on the left. No way to collect individual assignments #2354