MDL-56922 theme_boost: grey out activity icon when hidden

This was present in bootstrapbase and is I think an important visual
distinction.

theme/boost/scss/moodle/course.scss

@@ -89,6 +89,12 @@
                 text-indent: -31px;
                 padding-left: 31px;
             }
+
+            .dimmed {
+                .activityicon {
+                    opacity: .5;
+                }
+            }
         }
     }