Add index to create_at for track sent, so can remove old ones / only use new ones
user
```ruby
class AddIndicesTrackSent < ActiveRecord::Migration
    def self.up
        add_index :track_things_sent_emails, :created_at
    end

    def self.down
        remove_index :track_things_sent_emails, :created_at
    end
end
```
assistant