Send 7 day reminder email

[gl2748]

Previously: https://github.com/steemit/faucet-admin/issues/30

Crucially we want record that a reminder was sent for each use.

@jnordberg suggests:

Implement a cronjob in faucet that polls for state == approved && approved_at > 7 days && reminder_sent_at == null with the appropriate schema updates to allow for that query. If we deploy it 1 week from now that would mean that the 30k users who maybe got an email will receive another one if they still didn’t create their account but after that that we are sure of what we will be sending and it will be automatic not an admin action.

[pkattera]

Also needs to have an unsubscribe link