Get sections to render when switching datasource (#1660)

* Get sections to render when switching datasource
 - Move sectionsToRender in store and use is for defaultFormData
 - Change some SelectField to FreeFormSelect according to forms.py

* Solved the css not found problem in staging

* Fixed js tests

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

@@ -182,12 +182,14 @@ class ChartContainer extends React.Component {
               />
             </Alert>
           }
-          {!this.props.isChartLoading &&
-            <div
+          {this.props.isChartLoading ?
+            (<img alt="loading" width="25" src="/static/assets/images/loading.gif" />)
+            :
+            (<div
               id={this.props.containerId}
               ref={(ref) => { this.chartContainerRef = ref; }}
               className={this.props.viz_type}
-            />
+            />)
           }
         </Panel>
       </div>