quassel: Don't enable app when setup is rerun

Tests:

- Rerun setup after disabling the app. App is not enabled.

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

plinth/modules/quassel/__init__.py

@@ -98,7 +98,9 @@ class QuasselApp(app_module.App):
     def setup(self, old_version):
         """Install and configure the app."""
         super().setup(old_version)
-        self.enable()
+        if not old_version:
+            self.enable()
+
         self.get_component('letsencrypt-quassel').setup_certificates()