Twenty Twenty: Make CSS adjustments to account for the latest editor changes...

Twenty Twenty: Make CSS adjustments to account for the latest editor changes and ensure that group blocks work in expected ways.

Props williampatton, allancole, karmatosed, pierlo, joen, ianbelanger.
Fixes #48557.
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git-svn-id: https://core.svn.wordpress.org/trunk@46511 1a063a9b-81f0-0310-95a4-ce76da25c4cd