Refactor models files and db models conversions

[3shv]

Description

Related to issue 2110

Tried to remove the redundancy of full_order_into_model_order and made use of traits from std::convert.
Let me know if I should follow a different approach.

[3shv]

        // Filled - sell (filled - 100%)
        let status: OrderStatus = (&FullOrder {
            kind: DbOrderKind::Sell,
            sell_amount: BigDecimal::from(2),
            sum_sell: BigDecimal::from(3),
            sum_fee: BigDecimal::from(1),
            ..order_row()
        })
            .into();
        assert_eq!(status, OrderStatus::Fulfilled);

This test case is failing after refactor.. not sure why 🤔

Edit: Fixed after using sum_fee instead of executed_fee inside the function is_sell_order_filled