Fix issue with `request.user` in Celery task

When the user was not a "superuser", `request.user` was being
passed in the route object sent as a Celery job. This led to
an error (only reproducable in one of our instances, don't know
the exact conditions since all packets & versions seemed to be
identical):

    EncodeError: dictionary changed size during iteration

Fix:

* Use the `User` object retrieved from the ORM instead

Also, store the user even if the route is created from an admin.

flowspec/views.py

@@ -296,8 +296,7 @@ def add_route(request):
         form = RouteForm(request_data)
         if form.is_valid():
             route = form.save(commit=False)
-            if not request.user.is_superuser:
-                route.applier = request.user
+            route.applier = User.objects.get(username=request.user.username)
             route.status = "PENDING"
             route.response = "Applying"
             route.source = IPNetwork('%s/%s' % (IPNetwork(route.source).network.compressed, IPNetwork(route.source).prefixlen)).compressed