Add "soft_extended_due_date" field to Group

[james-perretta]

Group.soft_extended_due_date would behave similarly to Project.soft_closing_time. One key difference would be that Group.extended_due_date would remain visible to students in Group API objects. That way, users who are used to setting only one extension value can continue to do so. The UI will need some careful planning to accommodate this (or any other solution we come up with).

(Another proposed solution)
Adding a flag that makes a group's extension behave as a soft deadline gives us what we need for #523 but has the added benefit of being simpler for users who don't need this feature (they won't have to set 2 fields). It also is fully backwards compatible with no custom migrations.

(Old proposed solution)
This would be analogous to Project.soft_closing_time and eliminate some edge cases in #523.

A custom migration would be needed to initialize Group.soft_extended_due_date to the same value as Group.extended_due_date for each group.