Handle user re-subscribe

[karllhughes]

Problem

When users unsubscribe then re-subscribe we create a new record in the DB rather than "un-deleting" their old record.

New re-subscribe flow

• When user completely unsubscribes, it soft deletes them
• When deleted user signs up, "un-delete" that user and send new confirmation email
• Add unique index to email

[matthewtrask]

If no one else has taken this I think I can bang it out.