Bundled Themes: Twenty Twenty button styles produce inconsistent output and cannot be edited.

Fixes the issue with button styles not working when the Gutenberg plugin is activated while maintaining backwards compatibility.

Props nrqsnchz, poena, kjellr.
Fixes #49896.
Built from https://develop.svn.wordpress.org/trunk@47614


git-svn-id: https://core.svn.wordpress.org/trunk@47389 1a063a9b-81f0-0310-95a4-ce76da25c4cd