[security] prevent XSS markup viz (#3211)

* Prevent XSS in Markup viz

We protect the browser by sandboxing the user code inside an iframe

* Helvetica

superset/assets/javascripts/explore/components/controls/TextAreaControl.jsx

@@ -76,7 +76,7 @@ export default class TextAreaControl extends React.Component {
           modalTitle={controlHeader}
           triggerNode={
             <Button bsSize="small" className="m-t-5">
-              Edit <b>{this.props.language}</b> in modal
+              Edit <strong>{this.props.language}</strong> in modal
             </Button>
           }
           modalBody={this.renderEditor(true)}