Upgrade to React==16.4.1 & Enzyme==3.3.0 (#5359)

* Bumping react==16.4.1 &  enzyme==3.3.0

The upgrade was pretty smooth except for a cryptic message coming
out of react-select around running multiple copies of React. It turns
out the `common` bundle had React and was conflicting with explore and
dashboard apps, only in 16.x. This somehow wasn't a problem in 15.x
outside of the wasted resources.

Running 16.4 should bring in all sorts of perf improvements and features
we've all been waiting for.

https://reactjs.org/blog/2017/09/26/react-v16.0.html

TODO: react-bootstrap-datetimepicker isn't compatible with React 16

* Trying to deprecate react-bootstrap-datetime

* Moving forward

* Reintroducing tests

superset/assets/spec/javascripts/dashboard/components/gridComponents/Markdown_spec.jsx

@@ -113,9 +113,10 @@ describe('Markdown', () => {
     // the mode dropdown onchange instead
     const dropdown = wrapper.find(MarkdownModeDropdown);
     dropdown.prop('onChange')('preview');
+    wrapper.update();
 
-    expect(wrapper.find(AceEditor)).to.have.length(0);
     expect(wrapper.find(ReactMarkdown)).to.have.length(1);
+    expect(wrapper.find(AceEditor)).to.have.length(0);
   });
 
   it('should call updateComponents when editMode changes from edit => preview, and there are markdownSource changes', () => {