Create TaskAssignment table

[canjalal]

Currently:

Tasks belong to a single assignee but store due_date and trial_count.

Proposed:

Create AssignedTasks table. Each AssignedTask stores an assignee and a task and seen boolean. This allows a single task to be assigned to multiple assignees. All assignees can contribute toward the same trial_count.

Index on task_id
Uniqueness constraint on (task_id, assignee_id)