fix #1612 by adding robot.txt file

modoboa/core/templates/core/robots.txt

@@ -0,0 +1,2 @@
+User-Agent: *
+Disallow: /

modoboa/core/urls.py

@@ -1,6 +1,7 @@
 """Core urls."""
 
 from django.urls import path
+from django.views.generic.base import TemplateView
 
 from . import views
 
@@ -31,4 +32,5 @@ urlpatterns = [
     path('user/profile/', views.profile, name="user_profile"),
     path('user/api/', views.api_access, name="user_api_access"),
     path('user/security/', views.security, name="user_security"),
+    path('robots.txt', TemplateView.as_view(template_name="core/robots.txt", content_type="text/plain")),
 ]