Django Debug Toolbar: don't crash when not installed

It happens with some regularity that people notice the "DEBUG" setting
and try to run with DEBUG=True. Although this is not documented nor recommended
you can't really blame 'm, and it would probably help them debug their issues.

Pre-this-commit that was not possible, because the debug toolbar is usually not
installed (and on e.g. on Docker this is very annoying to do).

bugsink/urls.py

@@ -63,9 +63,16 @@ if settings.DEBUG:
         path('debug-users-email/<str:template_name>/', debug_users_email),
         path('debug-teams-email/<str:template_name>/', debug_teams_email),
         path('trigger-error/', trigger_error),
-        path("__debug__/", include("debug_toolbar.urls")),
     ]
 
+    try:
+        import debug_toolbar  # noqa
+        urlpatterns = [
+            path('__debug__/', include('debug_toolbar.urls')),
+        ] + urlpatterns
+    except ImportError:
+        pass
+
 
 handler400 = "bugsink.views.bad_request"
 handler403 = "bugsink.views.permission_denied"