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

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

Props mislavjuric, poena, audrasjb.
Fixes #49893.
Built from https://develop.svn.wordpress.org/trunk@47589


git-svn-id: https://core.svn.wordpress.org/trunk@47364 1a063a9b-81f0-0310-95a4-ce76da25c4cd