Create Sequelize Data models

[martoio]

Now that you have figured out how to talk to sequelize, let's try to convert the notes in datastructures.txt into the actual models that you can use in your application.

Let's also try to keep things 'production-ready' by using migrations instead of having the script update things automatically: https://sequelize.org/v5/manual/migrations.html

https://codeburst.io/sequelize-migrations-setting-up-associations-985d29b61ee7

If you find migrations a hard concept to grasp, then don't worry, we can skip them and just use regular DB operations.