Configure the application to deploy to Heroku

https://trello.com/c/0MQaFJpB

Previously, several warnings were being thrown by Heroku when deploying the application and assets were not being compiled correctly, which was slowing down the deployment of the application and causing the application to be rendered incorrectly. The Rails 12 Factor gem has been installed and images used in stylesheets have been updated to be included using the SASS helpers included in Rails.