Avoid rendering DayPicker when not visible

[lencioni]

I was looking at some markup that we generate during server rendering
and noticed a whole bunch of markup (~32 KiB of HTML) belonging to
react-dates for elements that were not visible. With a couple of minor
modifications, we can avoid rendering most of these nodes until the
input is actually focused and we want to open the DayPicker.

---

To @airbnb/webinfra @majapw

If you don't see any problems with this approach, I'll go ahead and update the tests.

[majapw]

I'll check out the branch and play around with it.

[majapw]

I'm seeing a proptype warning:

Warning: Failed prop type: The prop `children` is marked as required in `Portal`, but its value is `null`.
    in Portal (created by SingleDatePicker)
    in SingleDatePicker (created by SingleDatePickerWrapper)
    in SingleDatePickerWrapper
    in div (created by Story)
    in div (created by Story)
    in Story

I think this is likely because due to your change, this.renderDayPicker() sometimes returns null. Maybe that logic should be moved into this.maybeRenderDayPickerWithPortal() so as not to render the portal either in the case that the picker is hidden.

[majapw]

From playing around with the storybook, this looks and feels pretty good! We need to address the tests and also address the prop type warning as stated in my other comment, but over seems legit.

[lencioni]

@majapw Everything is updated. PTAL!

[coveralls]

Coverage decreased (-0.4%) to 86.501% when pulling 91f1ae4 on avoid-rendering-when-closed into 1deaa6c on master.