ES7 Migration - Linting

In an effort to keep the codebase manageable with numerous contributors, it would be helpful if we had a linter in place. The obvious choice is ESLint, followed by the specific style guide that we can start with and then customize accordingly.

I'll make some propositions:
* Airbnb styleguide: https://github.com/airbnb/javascript
* Prettier alongside the airbnb guide: https://github.com/prettier/prettier

Start by reviewing the repository's CoffeeScript codebase and the linked Airbnb JavaScript and Prettier proposals. Decide which linting approach fits the project, then define and apply a consistent configuration across the codebase. Done means the chosen linter is in place and the project follows its agreed style rules.

Written by the indexing model from the issue text.