Support More Kinds of Notifications

[tpjnorton]

ditto.

[tpjnorton]

Some ideas here:

For everyone

• Mentions on a task
• Comments on a task you're assigned to
• Updates to a task you're assigned to
• Being assigned to a task
• Being unassigned from a task

For admins / managers

• Plan expiring soon
• Payment failed
• Team Name
• Tasks becoming overdue (not assigned) (configurable)

[tpjnorton]

We also need to remove the unique constraint on notifications. It makes things a little simpler for now so we don't create double "Task Overdue" notifications.