Multiple queue, multiple cluster in one site (#71)

* Support for multi-queue, multi-cluster configuration. API changes include:
  * Adding `cluster` to async_task() parameters and Task model
  * Adding argument --name to qcluster command
  * Necessary adjustments to Conf and Broker classes
  * Some admin improvements

* Add settings.Q_CLUSTER['ALT_CLUSTERS']: q_cluster config overrides for alternative clusters;
Add Conf.CLUSTER_NAME: separate usage from Conf.PREFIX;
QueueAdmin/OrmQ detail page enhanced: now displaying args/kwargs/q_options instead of encrypted payload.

* if `cluster` argument is not set (the default), async_task() and schedule() will be handled by the default cluster; Documentation update.

* Text cleanup

* Documentation update.

* Fix TIMEOUT setting in Windows for non-default cluster

---------

Co-authored-by: Stan Triepels <1939656+GDay@users.noreply.github.com>

django_q/tests/test_scheduler.py

@@ -354,7 +354,7 @@ def test_scheduler(broker, monkeypatch):
     assert schedule.next_run.date() == (timezone.now() + timedelta(weeks=2)).date()
     broker.delete_queue()
 
-    monkeypatch.setattr(Conf, "PREFIX", "some_cluster_name")
+    monkeypatch.setattr(Conf, "CLUSTER_NAME", "some_cluster_name")
     # create a schedule on another cluster
     schedule = create_schedule(
         "math.copysign",
@@ -378,7 +378,7 @@ def test_scheduler(broker, monkeypatch):
     # queue must be empty
     assert task_queue.qsize() == 0
 
-    monkeypatch.setattr(Conf, "PREFIX", "default")
+    monkeypatch.setattr(Conf, "CLUSTER_NAME", "default")
     # create a schedule on the same cluster
     schedule = create_schedule(
         "math.copysign",