Make Welcome page into a simple React app (#4147)

* Make Welcome page into a simple React app

This removes a dependency on datatables, we should be able to get rid
of it as we re-write the Table and PivotTable viz

* tests/lint

* Bump node version to latest

superset/assets/spec/javascripts/welcome/App_spec.jsx

@@ -0,0 +1,22 @@
+import React from 'react';
+import { Panel, Col, Row } from 'react-bootstrap';
+import { shallow } from 'enzyme';
+import { describe, it } from 'mocha';
+import { expect } from 'chai';
+
+import App from '../../../javascripts/welcome/App';
+
+describe('App', () => {
+  const mockedProps = {};
+  it('is valid', () => {
+    expect(
+      React.isValidElement(<App {...mockedProps} />),
+    ).to.equal(true);
+  });
+  it('renders 2 Col', () => {
+    const wrapper = shallow(<App {...mockedProps} />);
+    expect(wrapper.find(Panel)).to.have.length(1);
+    expect(wrapper.find(Row)).to.have.length(1);
+    expect(wrapper.find(Col)).to.have.length(2);
+  });
+});