Disable primary-contact editing in console (#2745)

This is necessary because we'll use primary-contact emails as a way of
resetting passwords.

In the UI, don't allow editing of email address for primary contacts,
and don't allow addition/removal of the primary contact field
post-creation.

In the backend, make sure that all emails previously added still exist.

core/src/main/java/google/registry/ui/server/console/settings/ContactAction.java

@@ -169,6 +169,7 @@ public class ContactAction extends ConsoleApiAction {
         throw new ContactRequirementException(t);
       }
     }
+    enforcePrimaryContactRestrictions(oldContactsByType, newContactsByType);
     ensurePhoneNumberNotRemovedForContactTypes(oldContactsByType, newContactsByType, Type.TECH);
     Optional<RegistrarPoc> domainWhoisAbuseContact =
         getDomainWhoisVisibleAbuseContact(updatedContacts);
@@ -187,6 +188,23 @@ public class ContactAction extends ConsoleApiAction {
     checkContactRegistryLockRequirements(existingContacts, updatedContacts);
   }
 
+  private static void enforcePrimaryContactRestrictions(
+      Multimap<Type, RegistrarPoc> oldContactsByType,
+      Multimap<Type, RegistrarPoc> newContactsByType) {
+    ImmutableSet<String> oldAdminEmails =
+        oldContactsByType.get(Type.ADMIN).stream()
+            .map(RegistrarPoc::getEmailAddress)
+            .collect(toImmutableSet());
+    ImmutableSet<String> newAdminEmails =
+        newContactsByType.get(Type.ADMIN).stream()
+            .map(RegistrarPoc::getEmailAddress)
+            .collect(toImmutableSet());
+    if (!newAdminEmails.containsAll(oldAdminEmails)) {
+      throw new ContactRequirementException(
+          "Cannot remove or change the email address of primary contacts");
+    }
+  }
+
   private static void checkContactRegistryLockRequirements(
       ImmutableSet<RegistrarPoc> existingContacts, ImmutableSet<RegistrarPoc> updatedContacts) {
     // Any contact(s) with new passwords must be allowed to set them