Keep post publishing popover open when a date is clicked

[allilevine]

Description

Add a keepOpen prop to the DateTimePicker and keep focus on the date picker popover when true.

This isn't an ideal solution, given the note to remove keepFocusInside(), but removing it would require either changes to react-dates or replacing react-dates.

• Open react-dates issue to keep focus after user changes month: Body gets focus after user changes month with mouse click react-dates/react-dates#1065
• There's a prop isFocused we can pass to <DayPickerSingleDateController>, but the focus doesn't persist after transitioning to a different month.

To do: Open a follow-up issue to fix the popover changing position as the date changes (the popover position is relative to the publish date width).

Fixes #24206

How has this been tested?

1. Create a new post
2. Open the schedule popover by clicking on the publish date
3. Click on a date in the picker and confirm that the popover does not close.
4. Navigate to the next month
5. Click on a date and confirm that the popover does not close.
6. Navigate to a date via the keyboard and select it. Confirm that the popover does not close.

Screenshots

Gif of the popover staying open after clicking on a date:

Types of changes

Bug fix: adds a keepOpen prop to DateTimePicker and uses keepFocusInside() to keep the popover open when true.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR.

[github-actions]

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[retrofox]

LGTM. Although it might be fixed straight in the react-dates library, it seems that we aren't going to get a fix. We can re-iterate and update the code later in case the issue is fixed there.
Also, we've been considering replacing the component with other choices.
I'd like to know the opinion of other folks, too.
This PR fixes the issue, which is nice.

[tomjn]

This looks sensible, I'm giving this a thumbs up 👍🏻

[talldan]

Looks good, thanks for working on this, I renamed the PR to make it a bit more user-oriented, as the title ends up in the changelog for the release.

[allilevine]

I opened a follow-up issue to fix the popover position changing as the date changes: #29899