Bundled Themes: Twenty Twenty add icon for g.page links.

Adds the option to use a g.page url for the social menu.

Props argentite, mayankmajeji.
Fixes #49088.
Built from https://develop.svn.wordpress.org/trunk@47795


git-svn-id: https://core.svn.wordpress.org/trunk@47571 1a063a9b-81f0-0310-95a4-ce76da25c4cd