[explore-v2] make control panel sections and fields more dense (#1954)

* make control panel sections and fields more dense

* remove Panel

* use <Panel> with className prop

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

@@ -32,7 +32,10 @@ export default class ControlPanelSection extends React.Component {
 
   render() {
     return (
-      <Panel header={this.renderHeader()}>
+      <Panel
+        className="control-panel-section"
+        header={this.renderHeader()}
+      >
         {this.props.children}
       </Panel>
     );