dynamicdns: Split app initialization

Tests:

- When configured, after startup, Dynamic DNS domain shows up in names module.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: James Valleroy <jvalleroy@mailbox.org>

plinth/modules/dynamicdns/__init__.py

@@ -80,6 +80,9 @@ class DynamicDNSApp(app_module.App):
                                        **manifest.backup)
         self.add(backup_restore)
 
+    @staticmethod
+    def post_init():
+        """Perform post initialization operations."""
         current_status = get_status()
         if current_status['enabled']:
             domain_added.send_robust(sender='dynamicdns',