Issue #3091919 by owenbush, the_glitch: Registrant Settings needs a label field

...stall/recurring_events_registration.registrant.config.yml

...on/config/schema/recurring_events_registration.schema.yml

...events_registration/recurring_events_registration.install

...s_registration/recurring_events_registration.services.yml

...nts_registration/recurring_events_registration.tokens.inc

...s/recurring_events_registration/src/Entity/Registrant.php

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

...recurring_events_registration/src/NotificationService.php

...src/Plugin/Field/ComputedRegistrantTitleFieldItemList.php

...g_events_registration/src/RegistrationCreationService.php