Show clear and actionable query timeout error message (#2763)

* Show clear and actionable query timeout error message

1. Instead of waiting for a long time or server-side response 504 Gateway timeout, explore view now add a query timeout threshold. After timeout it will show specific querytimeout message.
2. fix alert box close button position.

* Show clear and actionable query timeout error message

1. Instead of waiting for a long time or server-side response 504 Gateway timeout, explore view now add a query timeout threshold. After timeout it will show specific querytimeout message.
2. fix alert box close button position.
3. fix a linting error.

superset/assets/javascripts/explorev2/components/ChartContainer.jsx

@@ -169,12 +169,14 @@ class ChartContainer extends React.PureComponent {
   renderAlert() {
     const msg = (
       <div>
-        {this.props.alert}
         <i
           className="fa fa-close pull-right"
           onClick={this.removeAlert.bind(this)}
           style={{ cursor: 'pointer' }}
         />
+        <p
+          dangerouslySetInnerHTML={{ __html: this.props.alert }}
+        />
       </div>);
     return (
       <div>