Switch to Kramdown

What

Right now we're using Redcarpet, but eventually we want to switch to Kramdown.

Any custom Redcarpet code we're currently using will need to be replaced (see lib/helpers_.rb). We'll also need to make sure everything still works with the new Markdown parser.

Why

With Kramdown we open a new world of possibilities in our docs site, making it as flexible as about.gitlab.com:

• This is our markdown guide for GitLab and the docs site
• This is our markdown guide for about.gitlab.com

Latest thread on Slack (2018-02-23): https://gitlab.slack.com/archives/C16HYA2P5/p1519332161000412

What can we do with Kramdown that we can't with Redcarpet

• Add videos, JS code, HTML code, apply custom classes and ids to markdown files
• Add the product badges inline to the docs (https://gitlab.com/gitlab-com/gitlab-docs/issues/173)
• Play with images and illustrations (e.g., add icons to headers https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/17429)
• Use FontAwesome icons in markdown files