Add prepend left and append right 15px helpers

app/assets/stylesheets/framework/common.scss

@@ -461,11 +461,13 @@ img.emoji {
 .prepend-left-4 { margin-left: 4px; }
 .prepend-left-5 { margin-left: 5px; }
 .prepend-left-10 { margin-left: 10px; }
+.prepend-left-15 { margin-left: 15px; }
 .prepend-left-default { margin-left: $gl-padding; }
 .prepend-left-20 { margin-left: 20px; }
 .append-right-5 { margin-right: 5px; }
 .append-right-8 { margin-right: 8px; }
 .append-right-10 { margin-right: 10px; }
+.append-right-15 { margin-right: 15px; }
 .append-right-default { margin-right: $gl-padding; }
 .append-right-20 { margin-right: 20px; }
 .append-bottom-0 { margin-bottom: 0; }