Hide After Due setting for Timed Exams

TNL-4366
Changes in studio to allow the hide_after_due setting to be utilized.

Includes:
    -python changes to contentstore, where the data is stored.
    -refactoring of timed-examination-preference-editor.underscore, to add this
        setting and make the editor more accessible.
    -javascript changes to link the new setting to the correct data model.
    -sass updates to fix a11y issues on the editor modal.
    -addition of post-due visibility information to course outline in studio.
    -new tests: python, js, acceptance, and a11y

common/lib/xmodule/xmodule/seq_module.py

@@ -97,6 +97,16 @@ class ProctoringFields(object):
         scope=Scope.settings,
     )
 
+    hide_after_due = Boolean(
+        display_name=_("Hide Exam Results After Due Date"),
+        help=_(
+            "This setting overrides the default behavior of showing exam results after the due date has passed."
+            " Currently only supported for timed exams."
+        ),
+        default=False,
+        scope=Scope.settings,
+    )
+
     is_practice_exam = Boolean(
         display_name=_("Is Practice Exam"),
         help=_(