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/DashboardTable_spec.jsx

@@ -0,0 +1,31 @@
+import React from 'react';
+import { mount } from 'enzyme';
+import { describe, it } from 'mocha';
+import { expect } from 'chai';
+
+import DashboardTable from '../../../javascripts/welcome/DashboardTable';
+
+const $ = window.$ = require('jquery');
+
+
+describe('DashboardTable', () => {
+  const mockedProps = {};
+  let stub;
+  beforeEach(() => {
+    stub = sinon.stub($, 'getJSON');
+  });
+  afterEach(() => {
+    stub.restore();
+  });
+
+  it('is valid', () => {
+    expect(
+      React.isValidElement(<DashboardTable {...mockedProps} />),
+    ).to.equal(true);
+  });
+  it('renders', () => {
+    const wrapper = mount(<DashboardTable {...mockedProps} />);
+    expect(stub.callCount).to.equal(1);
+    expect(wrapper.find('img')).to.have.length(1);
+  });
+});