Add WorkerActivity start time and average/min/max processing time

Refs #1394 (closed)

• A new WorkerActivity.started DateTime field is introduced.
• started is set to the existing updated DateTime for WorkerActivities that are Started at the time of the migration.
• A constraint ensures that we always set started when trying to update the activity to a started state.
• The complex QuerySet to retrieve the WorkerActivity statistics has been deduplicated from the CorpusWorkersActivity and ProcessWorkerActivity endpoints.
• ActivityManager.bulk_insert no longer accepts a state argument. queued was the only state that was actually used, and handling setting the start time when using state=started would have been a mess with the raw SQL query as well as completely useless, so I just removed the ability to create activities as started.