email: Fix Thunderbird auto configuration failure

- When there is no URL name provided, the view fails to render when computing
breadcrumbs from middleware. Provide a name so that the URL does not lead to a
500 HTTP error.

Tests:

- Accessing the URL on development machine does not lead to an error page.

- On a production machine, when trying to use Thunderbird account setup wizard,
without the patch, Thunderbird says that it found configuration by enumerating
common names for the domain. A 500 error can be seen in the journal. After the
change in patch, Thunderbird says that it found the configuration from the
provider. A 200 success code can be seen from journal.

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

plinth/modules/email/urls.py

@@ -16,5 +16,6 @@ urlpatterns = [
             name='dns'),
     path('apps/email/aliases/', non_admin_view(views.AliasView.as_view()),
          name='aliases'),
-    path('apps/email/config.xml', public(views.XmlView.as_view())),
+    path('apps/email/config.xml', public(views.XmlView.as_view()),
+         name='config-xml'),
 ]