Late Due Date Extend by delta

[cycomachead]

(Using this issue to test copilot)

The integrations for Canvas and Gradescope need updating. They need to set the late due date field when posting an extension.

• If the original assignment has no late due date, leave it empty
• otherwise shift the original late due date by the same delta as new due date.

Create new course setting, true by default, and add a script to back fill all old courses called "Extend Late Due Date Automatically"

• if this setting is off, AND the late due date is present on the original assignment then ensure the NEW late due date is the later of the original late due date and the approved extended due date. (i.e. make sure the new late due date is at least the same as or later than the new extended due date.)

Please add tests and suggest refactoring as necessary