Add CoffeeLint

What does this MR do?

• add Gem to use CoffeeLint
• add rake coffeelint to .gitlab-ci.yml
• add coffeelint.json from https://github.com/polarmobile/coffeescript-style-guide
• fix linting errors

Why was this MR needed?

• because nobody wants to drink unlinted coffee

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/13224#note_12537431