Truly disable all updates via WP admin UI

DISALLOW_FILE_MODS implicitly includes AUTOMATIC_UPDATER_DISABLED and
DISALLOW_FILE_EDIT, plus it hides the update notifications in the admin
bar and the "Updates" sub-menu in the "Dashboard" admin menu, and it
also hides "Update Available" messages in "Appearance -> Themes" and
"Plugins -> Installed Plugins".