Add wallet balance to User model and implement Atoll/Island management

- Added `wallet_balance` field to the User model.
- Updated UserAdmin to include `wallet_balance` in the admin interface.
- Created serializers and views for Atoll and Island management.
- Implemented endpoints for listing, creating, and updating Atolls and Islands.
- Enhanced payment processing with UUIDs for Payment and Topup models.
- Added migration files for new fields and constraints.
- Improved error handling and validation in various views.
- Updated email templates for better responsiveness and SEO.

billing/serializers.py

@@ -0,0 +1,11 @@
+from rest_framework import serializers
+from .models import Payment
+from devices.serializers import DeviceSerializer
+
+
+class PaymentSerializer(serializers.ModelSerializer):
+    devices = DeviceSerializer(many=True, read_only=True)
+
+    class Meta:
+        model = Payment
+        fields = "__all__"