feat: add reset option to Randomized Content Block

This makes the reset button to refresh the contents of a Randomized
Content Block (RCB) without reloading the full page by fetching a new
set of problems in the "reset" response and replacing the DOM contents.

The reset button returns the student view as a string and the client
uses the HtmlUtils package to replace the contents and reinitializes the
XBlock.

This allows students to use the RCB as a flash card system.

Co-authored-by: tinumide <tinuade@opencraft.com>

lms/static/sass/course/courseware/_courseware.scss

@@ -635,6 +635,17 @@ html.video-fullscreen {
         border-bottom: 1px solid #ddd;
         margin-bottom: ($baseline*0.75);
         padding: 0 0 15px;
+
+        .problem-reset-btn-wrapper {
+          position: relative;
+          .problem-reset-btn {
+            &:hover,
+            &:focus,
+            &:active {
+              color: $primary;
+            }
+          }
+        }
       }
 
       .vert > .xblock-student_view.is-hidden,