Add an option to add the webpack role into Drifter

[rhea-astons]

This adds an option do add the webpack role into Drifter. This also creates:

• the appropriate webpack config file for the Django setup
• an empty JS bundle as a starting point
• an empty main scss file as a starting point
• a README file with instructions for developers

[rhea-astons]

Added webpack as default front-end stack