feat: answer received form submissions by mail

.env

@@ -84,6 +84,10 @@ MYEP_OAUTH2_URL_RESOURCE_OWNER_DETAILS=https://my.ep-reisen.de/api/userinfo
 MYEP_OAUTH2_SCOPES=email,id,roles,profile
 # Shared key the sibling websites send in the X-Api-Key header. Empty here: set it per stage.
 APPLICATION_FORM_API_KEY=
+# Booking link for the Team-Infocall, substituted for {bookingUrl} in the application form
+# receipt mail. Empty here: set it per stage. While it is empty that mail is not sent at all -
+# an invitation with a dead booking link is worse than none.
+APPLICATION_FORM_BOOKING_URL=
 
 ###> symfony/mailjet-mailer ###
 # Bulk teamer mailings only, everything else goes out over MAILER_DSN. Set the real