coturn: Completely uninstall app

Tests:

* Verify from the UI that after reinstalling the app a new secret is generated
* Functional tests passed

Signed-off-by: nbenedek <contact@nbenedek.me>
[sunil: Update docstrings, make uninstall fail-safe]
Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: Sunil Mohan Adapa <sunil@medhas.org>

plinth/modules/coturn/__init__.py

@@ -114,6 +114,11 @@ class CoturnApp(app_module.App):
         self.get_component('letsencrypt-coturn').setup_certificates()
         notify_configuration_change()
 
+    def uninstall(self):
+        """De-configure and uninstall the app."""
+        super().uninstall()
+        privileged.uninstall()
+
 
 def get_available_domains():
     """Return an iterator with all domains able to have a certificate."""

plinth/modules/coturn/privileged.py

@@ -89,3 +89,9 @@ def augeas_load():
     aug.load()
 
     return aug
+
+
+@privileged
+def uninstall():
+    """Remove configuration file."""
+    CONFIG_FILE.unlink(missing_ok=True)