Bring in the widgets component of MP6. See #25858.

* A cleaner UI, with a simpler design for available and inactive widgets, and more emphasis on your sidebars.
* Available widgets and sidebars expand to two columns at larger screen sizes making it easier to manage sites with lots of sidebars.
* New dropzone indicator for empty sidebars.

Props shaunandrews, joen, iammattthomas.


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