fix scrolling on markup vis (#2644)

superset/assets/visualizations/markup.css

@@ -1,6 +1,5 @@
 .markup.slice_container {
     padding: 10px;
-    overflow: auto;
 }
 .separator {
     background-color: transparent !important;

superset/assets/visualizations/markup.js

@@ -4,6 +4,10 @@ require('./markup.css');
 
 function markupWidget(slice, payload) {
   $('#code').attr('rows', '15');
+  slice.container.css({
+    overflow: 'auto',
+    height: slice.container.height(),
+  });
   slice.container.html(payload.data.html);
 }