Mod8

## Server Setup
- Add `unicorn` gem and `config/unicorn.rb` file
- Add `Procfile` file

## Sidekiq Workers
- Add `sidekiq` gem
- Add `require 'sidekiq/testing'` and `Sidekiq::Testing.inline!` to `spec_helper.rb` 
- Refactor `UserMailer` to use sidekiq `delay` method

## Schema
- Add `admin` column to `User` model
- Test `admin?` user method