Post launch tasks

[iandunn]

#38 has all the pre-launch tasks for rolling the new header/footer out to all sites in the w.org network. This issue tracks the post-launch tasks.

• port documentation from old header.php and footer.php
• delete unnecessary functions like wporg_get_global_header() and vars like $wporg_global_header_options
• delete old /header.php so it doesn't get used in future
• remove wp-content/themes/pub/wporg/inc/header.php since it's not used?
• remove old header/footer styles from wp4.css (and -rtl)
• remove unused rosetta menu things like fetch_custom_header_data() unless still used by old bbpress? probably remove load_custom_header() and maybe others. leave wp_nav_menu_objects since it's being used
• remove $download from Rosetta_Sites::wp_nav_menu_objects()
• remove GlotPress customizations switch_theme_for_new_header()
• add mu-plugins repo to Learn env so header/footer works there. then delete old header/footer include and wporg_get_global_header()
• add mu-plugins repo to 5ftF env so header/footer works there. then delete old header/footer include and wporg_get_global_header()
• remove FEATURE_2021_GLOBAL_HEADER_FOOTER feature flag
• remove navigation.js from classic themes b/c not used anymore?