Enable unchecking of outcomes from modedit (and delete grades)

When editing a module that has outcomes associated with it, we should be able to uncheck these outcomes. When the form is submitted, these unchecked outcomes are deleted from the gradebook where they existed as grade_items.

However, if grades were already recorded in these outcomes, they will end up being deleted too. So appropriate warning should be given:

1. Visual label (warning icon like /!\ ) next to each outcome with grades, and a note explaining the consequences
2. A new checkbox below the outcomes: "Confirm deletion of grades"

Additionally, there could be a similar warning when a grade_item is about to be deleted from the gradebook.