Support recurring Tasks

[hugginsio]

The data model should be extended to support recurrence.

One option for this is to use a parent/child model, similar to Taskwarrior:

• Task status becomes StatusRecurring
• Task becomes a template with no DisplayID
• Spawn one instance ahead of current open task to keep clean up to Until date
• Child tasks have a Parent field to track the template ULID and an InstanceIndex integer for explicit ordering

Data model changes:

• Recur: Recurrence rule.
• Until: End date for recurring Tasks.
• Parent: TaskID of parent template for recurring Tasks.
• InstanceIndex: Tracks the recurrence instance.