Unit tests for bugs in IDDE API, including LMS-6563
Fix: an individual extended due date must not precede the normal due date LMS-6563
Fix: API allowed an individual due date extension on units with no due date
Fix: 500 response if indiv. due date deleted after normal due date was deleted
Fix: Display error when attempting to reset nonexistent due date extension
Add myself to AUTHORS, update changelog
Merge pull request #4868 from bradenmacdonald/fix-idde-bugs

Fixed: Can set earlier due date as an individual due date extension and related bugs