Bundled Themes: Twenty Twenty add WhatsApp to the social icon menu.

This adds WhatsApp as an option in the social icon menu.

Props rryyaanndd, audrasjb.
Fixes #49098.
Built from https://develop.svn.wordpress.org/trunk@47243


git-svn-id: https://core.svn.wordpress.org/trunk@47043 1a063a9b-81f0-0310-95a4-ce76da25c4cd