Add text input to InputDate

[emplums]

From Slack:

Wanted to bring up an accessibility thought around our date pickers! I think this is mentioned in Inclusive Design Patterns, but a common a11y pattern for date pickers is to also include a combo box or text input that users can type their date in directly - this lets users on screen readers skip having to keyboard navigate through all the dates in a month! We just got some more feedback about this from a blind user who is having a hard time scheduling posts in Buffer because our current Date Picker doesn’t include a combo box: https://secure.helpscout.net/conversation/359539095/1081589/

Here's an example of how this can be implemented in react-day-picker: http://react-day-picker.js.org/examples/?input

Here's Dave's mockup of what this might look like! :D