Add bullet gem and check for N+1 queries

[apoltieva]

As a developer, I want to be sure my queries are efficient. Steps:

• add bullet gem
• hit the endpoints that utilise db queries in the controller (that is, go to /cars page, /statistics page an so on)
• check for N+1 queries and modify the queries if needed
• give a short talk to the classmates about N+1 queries (less than 5 minutes, no presentation needed)