Support using celery task groups

[kotfic]

Deliverables

• Demonstrate syntax like the following from girder REST endpoint:

grp = celery.group([task.delay(item_id) for item_id in items])
grp.delay()

Technical Notes

• Note that celery.group is its own celery task which needs to have a job model
• This may require implementing something like a Group jobModels in girder
  • Not sure if this is strictly necessary - Celery implements a GroupResult object that has a similar API to AsyncResult but considers all the groups child tasks.

Depends on:

• Creation of girder job models via REST endpoints (Creation of girder job models via REST endpoints in worker plugin girder#2018)
• Clean up Celery 3 & 4 support in app.py (Clean up Celery 3 & 4 support in app.py  #139)
• Use custom subclass of celery.app.Celery to hijack AsyncResult, ResultSet, and GroupResult functions (Use custom subclass of celery.app.Celery to hijack AsyncResult, ResultSet, and GroupResult functions #147)