Added ability to contact all registrants of an event

...gistration/recurring_events_registration.links.action.yml

..._events_registration/recurring_events_registration.module

...egistration/recurring_events_registration.permissions.yml

...ts_registration/recurring_events_registration.routing.yml

...s_registration/recurring_events_registration.services.yml

...nts_registration/recurring_events_registration.tokens.inc

...ents_registration/src/Controller/RegistrantController.php

...es/recurring_events_registration/src/Form/ContactForm.php

...g_events_registration/src/Form/RegistrantSettingsForm.php

...recurring_events_registration/src/NotificationService.php