MDL-81300 theme_boost: Fix dnd overlay in course sections

Drag and drop overlay in course page was being overlapped by divider
buttons and quick edit links.
That happens because both elements have their own z-index context.
Add z-index to the drag and drop overlay elenment to fix this issue.

theme/boost/scss/moodle/course.scss

@@ -1377,6 +1377,11 @@ $activity-add-hover: theme-color-level('primary', -10) !default;
             width: 100%;
         }
     }
+
+    // Add new z-index context for dnd overlay higher divider buttons and quick edit links.
+    .overlay-preview {
+        z-index: 5;
+    }
 }
 
 .description .course-description-item {

theme/boost/style/moodle.css

@@ -29014,6 +29014,9 @@ span.editinstructions .alert-link {
   padding: 2rem;
   width: 100%;
 }
+.course-section .overlay-preview {
+  z-index: 5;
+}
 
 .description .course-description-item {
   font-size: 0.875em;

theme/classic/style/moodle.css

@@ -29014,6 +29014,9 @@ span.editinstructions .alert-link {
   padding: 2rem;
   width: 100%;
 }
+.course-section .overlay-preview {
+  z-index: 5;
+}
 
 .description .course-description-item {
   font-size: 0.875em;