fix: make new field in xblock json serializable and don't assign due dates to ORAs
feat: display warning message in Studio if the relative date input is more than 18 weeks for custom pacing in self paced course
fix: handle due dates for mix of ORA and non ORA problems under a subsection and other styling fixes
feat: add a minimum restriction for self paced courses due date editor input
fix: naming of warning id divs to be more specific and exclude children of ORA problems in setting due dates
test: extracting dates for a self paced course with custom pacing
test: frontend for self paced custom pacing modal in studio and clean up its backend tests
fix: remove an unused line when getting children of custom pacing subsection, reorganize testing for custom pacing
fix: more specific comments to testing for custom PLS and remove a test case course
fix: more cleanup for self paced custom pacing PLS backend tests
