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

Tests:

- Config files are all symlinks in /etc/
- Web interface works

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

debian/freedombox.maintscript

@@ -137,3 +137,4 @@ 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~
 rm_conffile /etc/apache2/conf-available/syncthing-plinth.conf 23.10~
+rm_conffile /etc/apache2/conf-available/transmission-plinth.conf 23.10~

plinth/modules/transmission/__init__.py

@@ -8,6 +8,7 @@ from django.utils.translation import gettext_lazy as _
 
 from plinth import app as app_module
 from plinth import cfg, frontpage, menu
+from plinth.config import DropinConfigs
 from plinth.daemon import Daemon
 from plinth.modules.apache.components import Webserver
 from plinth.modules.backups.components import BackupRestore
@@ -57,7 +58,7 @@ class TransmissionApp(app_module.App):
 
     app_id = 'transmission'
 
-    _version = 6
+    _version = 7
 
     DAEMON = 'transmission-daemon'
 
@@ -93,6 +94,11 @@ class TransmissionApp(app_module.App):
         packages = Packages('packages-transmission', ['transmission-daemon'])
         self.add(packages)
 
+        dropin_configs = DropinConfigs('dropin-configs-transmission', [
+            '/etc/apache2/conf-available/transmission-plinth.conf',
+        ])
+        self.add(dropin_configs)
+
         firewall = Firewall('firewall-transmission', info.name,
                             ports=['http', 'https',
                                    'transmission-client'], is_external=True)