feat: answer received form submissions by mail

.env.test

@@ -10,3 +10,5 @@ APP_TRUSTED_HOSTS='^localhost$'
 # loaded in the test environment - so the throwaway key the tests send belongs here.
 # Keep it in sync with ApplicationFormControllerTest::KEY.
 APPLICATION_FORM_API_KEY=test-application-form-api-key
+# The receipt mail refuses to render without it, and several tests send that mail.
+APPLICATION_FORM_BOOKING_URL=https://bookings.example.com/team-infocall