settings: Add can_set_topics_policy_group realm setting.

Added `can_set_topics_policy_group` realm setting to control
permission to change per-channel `topics_policy`.

zerver/migrations/0714_realm_can_set_topics_policy_group.py

@@ -0,0 +1,23 @@
+# Generated by Django 5.1.7 on 2025-03-29 11:55
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("zerver", "0713_remove_realm_mandatory_topics"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="realm",
+            name="can_set_topics_policy_group",
+            field=models.ForeignKey(
+                null=True,
+                on_delete=django.db.models.deletion.RESTRICT,
+                related_name="+",
+                to="zerver.usergroup",
+            ),
+        ),
+    ]