TP2000-1590 TaskWorkflow summary info support

[paulpepper-trade]

TP2000-1590 TaskWorkflow summary info support

Why

TaskWorkflow summary information is required to present a workflow's information in a Task-like way.

TaskWorkflow summary info must be presentable alongside Task info, in order to support at-a-glance list views that combine the two types of information.

What

This PR:

• Adds a summary Task instance to the Workflow model, referenced by its Workflow.summary_task field.
• Includes migrations to create the new field in the database and populate with dummy data. The dummy data is only a consideration while this work is in development, since workflows will be newly introduced and migrations squashed before moving to the master, main line.
• Adds three new filter functions to TaskQueryset: non_workflow(), workflow_summary() and top_level(). These should cater for the need to extract the Task instances that provide summary information for various views in a follow-up PR.
• Adds and updates model factories and fixtures for use in related tests.
• Provides unit tests for new queryset filtering.

Checklist

• Requires migrations? No
• Requires dependency updates? No