Implement Filtering By Trip Ticket Status

* Renaming TripTicket#claimed? to TripTicket#approved?
* Adding approved, claimed and unapproved scopes to TripTicket class
* Ensuring emty results are returned properly for TripTicket scopes approved, unapproved and unclaimed
* Adding TripTicket.filter_by_claim_status method
* Renaming TripTicket#claimed? to TripTicket#approved?
* Adding claim status filter to trip ticket filtering form