No user flow for rejecting Gastro-App Interessensbekundung

#### Issue description

In the Gastro-App, administration officials can assign a regulation to signups before sending an official registration form. If the validation of the initial signup shows that a registration is not possible there is no way to tell the user who has signing up that they are not considered for registration.

#### Suggestion

Add a status `registration_rejected` that results in a rejection email when using the "send registration form links" action on a signup object.