ALH: Add devise authentication.

After creating the rails app.. I told postgres there will be a user with
`createuser -s sales_tracker`. Then I did the following after adding the
Gem to the Gemfile and adding the configs to my app:
$ bundle install
$ rails g devise:install
$ rails g devise User
$ rake db:migrate

Lastly, I made a User in my console with just an email, pass, pass conf