Create Bookings Model, migrations, and seeders

[yoshelb]

• [x] Model name is Booking
• [x] table name is Bookings
• [x] table attributes: npx sequelize-cli model:generate --name Booking --attributes spotId:integer,userId:integer,startDate:date,endDate:date
• [x] find all constraints/ validations and add them to Model and migration
• [x] add options object to migrations and seeders
• [x] define associations
• [ ] add validations to Expressvalidator
• [ ] validator messages: "startDate": "startDate cannot be in the past", "endDate": "endDate cannot be on or before startDate"
• [x] create seeders

[yoshelb]

Comparing dates might cause problems later: https://appacademy.instructure.com/courses/314/pages/tips-on-comparing-dates-in-sequelize-2?module_item_id=56496