add admin flash messaging and styles

app/assets/stylesheets/pages/_error.scss

@@ -25,6 +25,16 @@
   background-color: #f39c12;
 }
 
+.success {
+  @extend .error;
+  background-color: $brand-success;
+}
+
+.notice {
+  @extend .error;
+  background-color: $brand-info;
+}
+
 [data-id="live-coder-finish-later"] {
   .warning {
     margin-top: 0;