Respect both no_reserve and reserve_unknown as potential reserve statuses

[sweir27]

This is related to: artsy/eigen#1835

Basically, in places like here:

eidolon/Kiosk/App/Models/SaleArtwork.swift

Lines 4 to 16 in fdd5981

	enum ReserveStatus: String {
	case ReserveNotMet = "reserve_not_met"
	case NoReserve = "no_reserve"
	case ReserveMet = "reserve_met"
	var reserveNotMet: Bool {
	return self == .ReserveNotMet
	}
	static func initOrDefault (rawValue: String?) -> ReserveStatus {
	return ReserveStatus(rawValue: rawValue ?? "") ?? .NoReserve
	}
	}

we want to assume that gravity could return no_reserve or reserve_unknown as a SaleArtwork.reserve_status (and both should be treated the same way we treat NoReserve today.