Dashboard: Introduce a "Try Gutenberg" callout.

To encourage more people to try Gutenberg, this new Dashboard box allows site users to easily install and try out Gutenberg.

Props pento, melchoyce, joen, karmatosed.
Fixes #41316.


Built from https://develop.svn.wordpress.org/trunk@41895


git-svn-id: https://core.svn.wordpress.org/trunk@41729 1a063a9b-81f0-0310-95a4-ce76da25c4cd