Datepicker maintain focus prop

[BenjaminWFox]

Fixes #2285

Additional description

Currently in the datepicker, interaction with the Input element moves focus to the Calendar. This PR adds an optional prop canInputMaintainFocus to the Datepicker which, when true, allows clicking & interacting with the input element without losing focus.

I'm unsure if solving this by adding an additional prop is acceptable, but there hadn't been any movement on this issue in the last month so I thought I would try. This could also be adapted into the new default behavior pretty easily.

I believe that with canInputMaintainFocus set to true all of the pattern bullet points are satisfied.

I added a story, but have not added a browser-test explicitly for this as I had trouble simulating the focus event. If that would be a requirement please let me know.

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[BenjaminWFox]

Sorry, this appears to have some commits from something I playing with some time ago. I will see if I can remove them from this PR, otherwise I will close this and submit a new PR.

ETA: junk commits removed.

[interactivellama]

Thank you so much for carrying this forward! @BenjaminWFox

Apologies for the delay on the feedback. I think this is good pattern. It can be the default without the prop. You can remove the new prop.

The primary issue I see is that explicitly opening the menu with the right side icon should still focus on the menu day. It is currently not. The focus is staying on the icon

Something like a menuOpenedByIcon prop on CalendarWrapper would work and only be true when the icon is clicked, but not when the rest of the input is clicked into.

And please run npm run lint:fix to lint code. https://travis-ci.org/salesforce/design-system-react/jobs/606556443#L7438

Here is a screenshot:

[interactivellama]

Please see request left in previous comment.

[BenjaminWFox]

@interactivellama This is updated per your suggestions, let me know if any further edits are needed, thanks.

[interactivellama]

I pulled down and viewed in Storybook and the UX pattern is looking great! Give me a few days to look over the code and we should be good.

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[interactivellama]

Thank you @BenjaminWFox for your work!