[Fixes #946] Move focus to datepicker when SingleDatePicker is set to readOnly

[aporlando]

Issue #946 proposes that when SingleDatePicker is set to readOnly, then its onFocus behavior should move the focus to the datepicker from the input (since there's no opportunity to type in a readOnly input).
This helps accomplish behavior to hook into subsequent clicks on a readOnly input to close the datepicker (on non-touch enabled devices).
Note that this is a separate issue from #958 which provides a hook to keep focus on the input.

[coveralls]

Coverage remained the same at 86.329% when pulling 999ba3b on aporlando:onFocusReadOnly into 05b17b0 on airbnb:master.

[coveralls]

Coverage remained the same at 86.329% when pulling 999ba3b on aporlando:onFocusReadOnly into 05b17b0 on airbnb:master.

[coveralls]

Coverage remained the same at 86.329% when pulling 66d01ed on aporlando:onFocusReadOnly into 05b17b0 on airbnb:master.

[majapw]

Does this actually end up closing the datepicker when you click on the input again?

[aporlando]

No. It allows you to accomplish it in the onFocusChange event for readOnly inputs. Something like:

<SingleDatePicker readOnly
focused={this.state.focused}
onFocusChange={({ focused }) => {
  if (this.state.focused && focused) {
      // Already focused, so datepicker is open, focus event again means subsequent input interaction
      this.setState({ focused: false });
  }
  else {
      this.setState({focused});
  }
}}
/>

[coveralls]

Coverage remained the same at 84.85% when pulling 5169d47 on aporlando:onFocusReadOnly into 547e06f on airbnb:master.