radicale: Fix backup and restore of configuration

Perhaps the configuration was earlier stored under /var and symlinked from /etc/
but currently it is a separate directory and needs to be backed up.

Tests:
- Run functional tests for the radicale.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: Veiko Aasa <veiko17@disroot.org>

plinth/modules/radicale/manifest.py

@@ -79,6 +79,9 @@ clients = [{
 }]
 
 backup = {
+    'config': {
+        'directories': ['/etc/radicale/']
+    },
     'data': {
         'directories': ['/var/lib/radicale/']
     },