[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/index.jsx

@@ -14,6 +14,7 @@ import ExploreViewContainer from './components/ExploreViewContainer';
 import { exploreReducer } from './reducers/exploreReducer';
 import { appSetup } from '../common';
 import './main.css';
+import '../../stylesheets/reactable-pagination.css';
 
 appSetup();
 initJQueryAjax();