DEV: use new 'ignore allowed groups' site settings (#27670)

Instead of the deprecated 'min trust level to allow ignore' in order to reduce the number of deprecation notices in the logs.

This tweaks a few serializers so that the 'can_ignore_users?` property is always coming from the server and properly used on the client-side.

spec/requests/api/schemas/json/user_get_response.json

@@ -47,9 +47,15 @@
         "can_ignore_user": {
           "type": "boolean"
         },
+        "can_ignore_users": {
+          "type": "boolean"
+        },
         "can_mute_user": {
           "type": "boolean"
         },
+        "can_mute_users": {
+          "type": "boolean"
+        },
         "can_send_private_messages": {
           "type": "boolean"
         },