Add notifications for new sign-ups (#16953)

app/controllers/api/web/push_subscriptions_controller.rb

app/javascript/mastodon/actions/notifications.js

...odon/features/notifications/components/column_settings.js

...astodon/features/notifications/components/notification.js

app/javascript/mastodon/reducers/settings.js

app/javascript/mastodon/service_worker/web_push_locales.js

app/models/notification.rb

app/services/bootstrap_timeline_service.rb

app/services/notify_service.rb

config/i18n-tasks.yml

config/locales/en.yml