[slices] add simple new slice form (#2800)

* initial structure for add new slice page

* simplify add slice form

* add a test

* fix long line

* use underscore for template name

* fix controls path

* fix vis types select

superset/assets/javascripts/addSlice/index.jsx

@@ -0,0 +1,14 @@
+import React from 'react';
+import ReactDOM from 'react-dom';
+import { appSetup } from '../common';
+import AddSliceContainer from './AddSliceContainer';
+
+appSetup();
+
+const addSliceContainer = document.getElementById('js-add-slice-container');
+const bootstrapData = JSON.parse(addSliceContainer.getAttribute('data-bootstrap'));
+
+ReactDOM.render(
+  <AddSliceContainer datasources={bootstrapData.datasources} />,
+  addSliceContainer,
+);