avatar: Use fixed avatars for system bots.

This makes a Zulip server more isolated than relying on gravatar, and
avoids complex logistics if in the future we move system bots to live
inside individual realms.

Co-authored-by: PieterCK <pieterceka123@gmail.com>

zerver/migrations/0615_system_bot_avatars.py

@@ -0,0 +1,46 @@
+# Generated by Django 5.0.9 on 2024-10-14 23:22
+
+from django.conf import settings
+from django.db import migrations
+from django.db.backends.base.schema import BaseDatabaseSchemaEditor
+from django.db.migrations.state import StateApps
+
+
+def set_system_bot_avatar_source_user(
+    apps: StateApps, schema_editor: BaseDatabaseSchemaEditor
+) -> None:
+    UserProfile = apps.get_model("zerver", "UserProfile")
+    UserProfile.objects.filter(
+        email__in=[
+            settings.EMAIL_GATEWAY_BOT,
+            settings.NOTIFICATION_BOT,
+            settings.WELCOME_BOT,
+        ]
+    ).update(avatar_source="U")
+
+
+def set_system_bot_avatar_source_gravatar(
+    apps: StateApps, schema_editor: BaseDatabaseSchemaEditor
+) -> None:
+    UserProfile = apps.get_model("zerver", "UserProfile")
+    UserProfile.objects.filter(
+        email__in=[
+            settings.EMAIL_GATEWAY_BOT,
+            settings.NOTIFICATION_BOT,
+            settings.WELCOME_BOT,
+        ]
+    ).update(avatar_source="G")
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("zerver", "0614_remove_realm_move_messages_between_streams_policy"),
+    ]
+
+    operations = [
+        migrations.RunPython(
+            set_system_bot_avatar_source_user,
+            elidable=True,
+            reverse_code=set_system_bot_avatar_source_gravatar,
+        ),
+    ]