UX: makes reports fade in when loading is done (#6440)

app/assets/stylesheets/common/components/conditional-loading-section.scss

@@ -13,4 +13,27 @@
       font-weight: 700;
     }
   }
+
+  &:not(.is-loading) {
+    animation: fadein 0.5s;
+    -webkit-animation: fadein 0.5s;
+  }
+
+  @keyframes fadein {
+    from {
+      opacity: 0;
+    }
+    to {
+      opacity: 1;
+    }
+  }
+
+  @-webkit-keyframes fadein {
+    from {
+      opacity: 0;
+    }
+    to {
+      opacity: 1;
+    }
+  }
 }