sso: Use drop-in config component for /etc files

Tests:

- Config files are all symlinks in /etc/
- Single-sing-on for searx works

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

debian/freedombox.maintscript

@@ -135,3 +135,4 @@ rm_conffile /etc/apache2/includes/freedombox-sharing.conf 23.10~
 rm_conffile /etc/fail2ban/jail.d/ssh-freedombox.conf 23.10~
 rm_conffile /etc/avahi/services/sftp-ssh.service 23.10~
 rm_conffile /etc/avahi/services/ssh.service 23.10~
+rm_conffile /etc/apache2/includes/freedombox-single-sign-on.conf 23.10~

plinth/modules/sso/__init__.py

@@ -4,6 +4,7 @@
 from django.utils.translation import gettext_lazy as _
 
 from plinth import app as app_module
+from plinth.config import DropinConfigs
 from plinth.package import Packages
 
 from . import privileged
@@ -14,7 +15,7 @@ class SSOApp(app_module.App):
 
     app_id = 'sso'
 
-    _version = 1
+    _version = 2
 
     def __init__(self):
         """Create components for the app."""
@@ -31,6 +32,11 @@ class SSOApp(app_module.App):
         ])
         self.add(packages)
 
+        dropin_configs = DropinConfigs('dropin-configs-sso', [
+            '/etc/apache2/includes/freedombox-single-sign-on.conf',
+        ])
+        self.add(dropin_configs)
+
     def setup(self, old_version):
         """Install and configure the app."""
         super().setup(old_version)