Create Flight model & create a post endpoint for creation

[lothemar]

Fields:

• String flightNo unique
• Date departureTime
• Date arrivalTime
• String departureLocation
• String arrivalLocation
• Number seatsEcon
• Number seatsBus
• Number seatsFirst
• Array terminals

Necessary validators

• departureTime > arrivalTime
• seatsEcon, seatsBus, seatsFirst >= 0
• departureLocation != arrivalLocation

endpoint should be RESTful so the route should look like this: /flights and it should accept POST requests as described in API schema

[MoKamall]

Add Number price to the schema.