direct_message_group: Add new group_size field.

This commit adds a new `group_size` field to the `DirectMessageGroup`
model, and backfills its value to each of the existing direct message
groups.

Fixes part of #25713

zerver/migrations/0573_directmessagegroup_group_size.py

@@ -0,0 +1,17 @@
+# Generated by Django 5.0.6 on 2024-08-16 06:36
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("zerver", "0572_alter_usergroup_can_manage_group"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="directmessagegroup",
+            name="group_size",
+            field=models.IntegerField(null=True),
+        ),
+    ]