feat: add shimmer background when image loads

_sass/addon/module.scss

@@ -90,6 +90,10 @@
   font-style: normal;
 }
 
+%img-rounded {
+  border-radius: 10px;
+}
+
 %img-caption {
   + em {
     display: block;
@@ -114,6 +118,11 @@
   text-decoration: none;
 }
 
+@mixin mt-mb($value) {
+  margin-top: $value;
+  margin-bottom: $value;
+}
+
 @mixin ml-mr($value) {
   margin-left: $value;
   margin-right: $value;