To encourage folks to prepare for Gutenberg, this new Dashboard box allows site users to easily install and try the Gutenberg plugin now, or to install the Classic Editor plugin before WordPress 5.0 is released.
Props pento, melchoyce, joen, karmatosed, joemcgill, SergeyBiryukov, jorbin, bph, Clorith, afercia, chanthaboune, chrislema, kjellr, matveb, michelleweber.
Fixes#41316.
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