Fix styles in Separator widget (#3309)

Style wasn't working right for separator widget since the iframe sandboxing of
the markup widget. This addresses this small issue and also now allows
for html in the separator widget

superset/assets/javascripts/explore/stores/visTypes.js

@@ -358,6 +358,7 @@ export const visTypes = {
       {
         label: null,
         controlSetRows: [
+          ['markup_type'],
           ['code'],
         ],
       },