email_server: domains: Add button for domain management form

- It will dropped from tabs.

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

plinth/modules/email_server/templates/email_server.html

@@ -9,6 +9,10 @@
 {% endblock %}
 
 {% block subsubmenu %}
+  <a class="btn btn-default" role="button"
+     href="{% url 'email_server:domains' %}">
+    {% trans "Domains" %}
+  </a>
   <a class="btn btn-default" role="button" href="/rspamd/">
     {% trans "Manage Spam" %}
     <span class="fa fa-external-link"></span>

plinth/modules/email_server/urls.py

@@ -8,7 +8,8 @@ from . import views
 
 urlpatterns = [
     path('apps/email_server/', views.EmailServerView.as_view(), name='index'),
-    path('apps/email_server/domains', views.DomainView.as_view()),
+    path('apps/email_server/domains', views.DomainView.as_view(),
+         name='domains'),
     path('apps/email_server/my_mail',
          non_admin_view(views.MyMailView.as_view()), name='my_mail'),
     path('apps/email_server/my_aliases',