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/brokers/orm.py

@@ -16,7 +16,7 @@ def _timeout():
 
 class ORM(Broker):
     @staticmethod
-    def get_connection(list_key: str = Conf.PREFIX):
+    def get_connection(list_key: str = None):
         if transaction.get_autocommit(
             using=Conf.ORM
         ):  # Only True when not in an atomic block
@@ -55,8 +55,9 @@ class ORM(Broker):
         self.delete(task_id)
 
     def enqueue(self, task):
+        # list_key might be null (e.g. in a test setup) but OrmQ.key has not-null constraint
         package = self.get_connection().create(
-            key=self.list_key, payload=task, lock=timezone.now()
+            key=self.list_key or Conf.CLUSTER_NAME, payload=task, lock=timezone.now()
         )
         return package.pk