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

cms/static/sass/elements/_modal-window.scss

@@ -564,7 +564,7 @@
     }
     .list-fields {
       .field-message {
-        color: $gray;
+        color: $gray-d1;
         font-size: ($baseline/2);
       }
       .field {

cms/static/sass/elements/_modules.scss

@@ -303,7 +303,7 @@ $outline-indent-width: $baseline;
 %outline-item-status {
   @extend %t-copy-sub2;
   @extend %t-strong;
-  color: $color-copy-base;
+  color: $gray-d1;
 
   .icon {
     @extend %t-icon5;
@@ -576,12 +576,16 @@ $outline-indent-width: $baseline;
       > .subsection-status .status-timed-proctored-exam {
         opacity: 1.0;
       }
+
+      > .subsection-status .status-hide-after-due {
+        opacity: 1.0;
+      }
     }
 
     // status - grading
-    .status-grading, .status-timed-proctored-exam  {
+    .status-grading, .status-timed-proctored-exam, .status-hide-after-due {
       @include transition(opacity $tmg-f2 ease-in-out 0s);
-      opacity: 0.65;
+      opacity: 0.75;
     }
 
     .status-grading-value, .status-proctored-exam-value {