Add micropost migration, model, and basic tests - 10.1.1
Add more tests for micropost model - 10.1.2
Add User/Micropost associations and tests - 10.1.3
10.1.4
10.1.5
10.2.1
10.2.2
10.3.1
10.3.2
10.3.3
Add user microposts - Chapter 10