Add CycleTaskGroupObject.object to fix cache expiration

* Add migration, mixin, client-side attributes, and cache linkage for
  `CycleTaskGroupObject.object`
Update migration to fix existing CycleTaskGroupObjects
Add Cycle.is_current attribute

* Add migration and attribute; update filters/UI to use `is_current`
  instead of `status`.
get_state should only consider current cycles
Clear objects from memcache when updating a cycle
Enable more efficient query for expired objects in Cycle

* `get_cache_key` only requires `type` and `id`, so enable other
  representations of that data (tuple or dict)
* `Cycle.cycle_task_group_object_objects_for_cache` then only needs
  `object_type` and `object_id`, so no need to query for the full
  `.object` instance
Fix for more efficient query
Merge pull request #1558 from reciprocity/feature/fix-cycle-task-group-object-cache-expiration

Fix Cycle cache expiration and other issues