upgrades: Split app initialziation

Tests:

- Upon startup, upgrade check runs every 3 minutes.

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

plinth/modules/upgrades/__init__.py

@@ -92,6 +92,8 @@ class UpgradesApp(app_module.App):
                                        **manifest.backup)
         self.add(backup_restore)
 
+    def post_init(self):
+        """Perform post initialization operations."""
         self._show_new_release_notification()
 
         # Check every day (every 3 minutes in debug mode):