Add sign-up button to logged-out web UI (#19250)

app/javascript/mastodon/initial_state.js

@@ -14,6 +14,7 @@ export const deleteModal = getMeta('delete_modal');
 export const me = getMeta('me');
 export const searchEnabled = getMeta('search_enabled');
 export const limitedFederationMode = getMeta('limited_federation_mode');
+export const registrationsOpen = getMeta('registrations_open');
 export const repository = getMeta('repository');
 export const source_url = getMeta('source_url');
 export const version = getMeta('version');