email_server: clients: Launch roundcube directly instead of app page

- Maintain consistency with rest of the apps and do what the user will mostly
expect.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: Fioddor Superconcentrado <fioddor@gmail.com>

plinth/modules/email_server/manifest.py

@@ -8,7 +8,7 @@ clients = [
         'name': _('Roundcube'),
         'platforms': [{
             'type': 'web',
-            'url': '/plinth/apps/roundcube'
+            'url': '/roundcube/'
         }]
     },
     {