Use the correct iframe width with resize buttons. Fix #226 (#225)

Author: jkevingutierrez

pattern_library/static/pattern_library/src/scss/components/_iframe.scss

@@ -1,4 +1,5 @@
 .iframe {
+    box-sizing: content-box;
     width: 100%;
     border: 1px solid $light-grey;
     margin: 20px 0;