Add code analysis for JS code

[DavidHernandez]

Right now there is no clear code standard for JS. We should adopt one and add some code analysis to Travis to ensure we are following the decided standard.

@AronNovak suggested starting with the Drupal Code Standard for javascript and seems a sensitive approach to me: http://cgit.drupalcode.org/drupal/tree/core/.eslintrc?h=8.1.x