Support chaining celery tasks

[kotfic]

Deliverables:

• Demonstrate syntax like the following from girder REST endpoint:

	(task1.s () | task2.s()).delay()

• Demonstrate asynchronously calling a girder worker task from another girder worker task (e.g. like taskflow - allows for conditionals etc).

Technical Notes:

• Note on dependencies:
  • We need a REST endpoint for creating child jobs because in the case of a chain, the second task is actually “produced” from a girder_worker instance.
  • Parent child relationships need to exist inside girder because otherwise there is no way to visually relate chains of tasks
  • Celery 3 has no clear way to relate one task to another. To allow chaining we need Celery 4 to be the default

Depends on:

• Creation of girder job models via REST endpoints in worker plugin (Creation of girder job models via REST endpoints in worker plugin girder#2018)
• Allow parent-child relationships between girder job models ( Allow parent-child relationships between girder job models girder#2017)
• Clean up Celery 3 & 4 support in app.py (Clean up Celery 3 & 4 support in app.py  #139)