settings: Add setting for who can edit user groups.

Fixes #12380.

zerver/migrations/0252_realm_user_group_edit_policy.py

@@ -0,0 +1,20 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.24 on 2019-10-16 22:48
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('zerver', '0251_prereg_user_add_full_name'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='realm',
+            name='user_group_edit_policy',
+            field=models.PositiveSmallIntegerField(default=1),
+        ),
+    ]