upgrade to partitions without a costly bulk data migration

keep pre-upgrade events in an old table (instead of a partition)

- instead of creating a default partition, keep all events in special
"unpartitioned" tables
- track these tables via distinct proxy=true models
- when generating the queryset for a UnifiedJob's events, look at the
  creation date of the job; if it's before the date of the migration,
  query on the old unpartitioned table, otherwise use the more modern table
  that provides auto-partitioning

awx/main/models/inventory.py

@@ -35,7 +35,7 @@ from awx.main.fields import (
 )
 from awx.main.managers import HostManager
 from awx.main.models.base import BaseModel, CommonModelNameNotUnique, VarsDictProperty, CLOUD_INVENTORY_SOURCES, prevent_search, accepts_json
-from awx.main.models.events import InventoryUpdateEvent
+from awx.main.models.events import InventoryUpdateEvent, UnpartitionedInventoryUpdateEvent
 from awx.main.models.unified_jobs import UnifiedJob, UnifiedJobTemplate
 from awx.main.models.mixins import (
     ResourceMixin,
@@ -1265,6 +1265,8 @@ class InventoryUpdate(UnifiedJob, InventorySourceOptions, JobNotificationMixin,
 
     @property
     def event_class(self):
+        if self.has_unpartitioned_events:
+            return UnpartitionedInventoryUpdateEvent
         return InventoryUpdateEvent
 
     @property