Add Cars to User

[apoltieva]

The whole point of our web app is to have car information and support.

ACs:

• cars#index is the root (it shows all their cars)
• on the home page there exists a button 'Add a new car' which redirects a user to a form where they can add a new car
• a car belongs to exactly one user
• car model has attributes described in the entity-relation diagram
• seed the database with test data (user and car, tip: use faker gem)