Enable hot module replacement for React via react-hot-loader (#5841)

* Install react-hot-loader

* enable react-hot-reload for Explore, Dashboard, Profile and Sqllab

* enable hmr for welcome page

* enable hmr for welcome page

* enable react hot module replacement for addSlice

* fix lint

* fix Welcome test

* remove eslint comment

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

@@ -0,0 +1,22 @@
+import React from 'react';
+import { Panel, Row, Tab } from 'react-bootstrap';
+import { shallow } from 'enzyme';
+import { describe, it } from 'mocha';
+import { expect } from 'chai';
+
+import Welcome from '../../../src/welcome/Welcome';
+
+describe('Welcome', () => {
+  const mockedProps = {};
+  it('is valid', () => {
+    expect(
+      React.isValidElement(<Welcome {...mockedProps} />),
+    ).to.equal(true);
+  });
+  it('renders 4 Tab, Panel, and Row components', () => {
+    const wrapper = shallow(<Welcome {...mockedProps} />);
+    expect(wrapper.find(Tab)).to.have.length(3);
+    expect(wrapper.find(Panel)).to.have.length(3);
+    expect(wrapper.find(Row)).to.have.length(3);
+  });
+});