Authenticate with GitHub

We must complete the following sub-tasks to complete Step 2.

[x] Create and configure a new GitHub Apps application.
[x] Assign "Email addresses - read-only" in the "User permissions" section.
[x] Generate a model User with properties name and email.
[x] Add Omniauth gems: gem 'omniauth-github' and gem 'omniauth-rails_csrf_protection'.
[x] Add the initializer: "config/initializers/omniauth.rb".
[x] Add route handlers to "config/routes.rb".
[ ] Create "app/controllers/web/auth_controller.rb".
[ ] Add a navigation link to sign in/sign out.
[ ] Implement the callback() method for the authentication.
[x] Configure OmniAuth test mode.
[x] Add test helpers for authentication.
[x] Add tests for the auth controller.
[ ] Make sure that the tests pass.
[x] Use gem 'dotenv-rails' to set environment variables in the development environment.
[x] Create file ".env.example" and add .env to the ".gitignore" file.
[x] Generate a new client secret.
[ ] Add a Makefile command to create ".env" from ".env.example" and populate it with the provided values.
[ ] Check that the authentication works in the local instance.
[ ] Set the environment variables in the production dashboard.
[ ] Check that the authentication works in the production instance.

yurivyatkin / rails-project-65