feat: AA-885 show offsets in studio self paced course outline for a subsection

feat: AA-883 basic prototype for custom pacing pls in studio

refactor: merge with basic prototype for self paced courses from AA-844

feat: add due date estimate message in self paced courses studio modal

refactor: merge with main that has up to date self paced custom pls editor and tests

fix: only display projected date if start date exists

fix: tests to check grading date in outline

fix: only one warning message show at a time

fix: do not show projected date when it is before the start date