Adding date form to date picker and improving accessibility (#7621)

* Date form has been added to the date-time component, as a primary input method
* Locale-dependent field order, with en-US being month-day, and the default day-month
* Keyboard focus goes to the month selector first, as dates are validated based on the month (and year if leap-year)
* The post-schedule sidebar widget is now more accessible, with button designated as a live object, so changes should be announced to date pickers
* React-Datepicker widget has been removed from the keyboard navigation and screenreader context, as the new form fields are not the primary input method.
* Tiny bit of code cleanup

lib/client-assets.php

@@ -344,9 +344,10 @@ function gutenberg_register_scripts_and_styles() {
 						),
 					),
 					'formats'  => array(
-						'time'     => get_option( 'time_format', __( 'g:i a', 'default' ) ),
-						'date'     => get_option( 'date_format', __( 'F j, Y', 'default' ) ),
-						'datetime' => __( 'F j, Y g:i a', 'default' ),
+						'time'                => get_option( 'time_format', __( 'g:i a', 'default' ) ),
+						'date'                => get_option( 'date_format', __( 'F j, Y', 'default' ) ),
+						'datetime'            => __( 'F j, Y g:i a', 'default' ),
+						'datetimeAbbreviated' => __( 'M j, Y g:i a', 'default' ),
 					),
 					'timezone' => array(
 						'offset' => get_option( 'gmt_offset', 0 ),