Display first name and last initial for user reviews

**Collect users' first and last name during sign up:**
* Create migration for adding first and last names
* Validate presence of first and last name in `User`
* Add first and last names to `users#new` and `UsersController`

https://trello.com/c/ZDsDcfOt

**Show first name and last initial for user reviews:**
* Extract helper method to format user's name like "John S."

https://trello.com/c/SxaK2Bld
Space missing after comma.