Use JavaScript tool chain for frontend development

This is very meta and rather a discussion related to many issues than an issue itself.

I suggest using node.js for

• dependency resolution (#14634 (closed), #20675 (closed))
• translating SASS into CSS (#18432 (closed))
• running JavaScript tests (!5787 (closed))
• calculating JavaScript test coverage (!5052 (merged))
• linting JavaScript (!5445 (merged))

We already use node.js and we already use JavaScript tools wrapped in a Gem. However

• some tools have no Gem, like istanbul which is required by teaspoon for test coverage
• some tools have a wrapper Gem which is not straightforward to use
• wrappers are by nature always behind the current development (depending on the maintainer that may be critical)
• wrappers may introduce problems which are not in the underlying tool
• wrappers may have a simplified API missing on parameters (e.g. https://gitlab.com/gitlab-org/gitlab-ce/issues/20757#note_13735035)
• people writing JavaScript know JavaScript but not necessarily Ruby
• Rails is not a JavaScript framework

What do you think, frontenders? 😃 @jschatz1 @alfredo1 @iamphill @annabeldunstone @fatihacet @connorshea @lbennett