[explore] DatasourceControl to pick datasource in modal (#3210)

* [explore] DatasourceControl to pick datasource in modal

Makes it easier to change datasource, also makes it such that the list
of all datasources doesn't need to be loaded upfront.

* Adding more metadata

superset/assets/javascripts/explore/components/ExploreViewContainer.jsx

@@ -33,9 +33,6 @@ class ExploreViewContainer extends React.Component {
   }
 
   componentDidMount() {
-    if (!this.props.standalone) {
-      this.props.actions.fetchDatasources();
-    }
     window.addEventListener('resize', this.handleResize.bind(this));
     this.triggerQueryIfNeeded();
   }