Add support for embedded videos

• This MR introduces support for embedded videos with a fallback for /help. Thumbnails and special styling can come later.
• Follow-up from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29683.
• Implementation per Mike's requested on Slack:

@mikelewis:

@marcia I think we should have the capability to easily embed videos like that, and also create some kind of expanding video link that uses a video icon or thumbnail.

Review App: http://embed-videos.178.62.207.141.xip.io/ce/development/documentation/styleguide.html#embedded-videos

Todos:

• Roll back .gitlab-ci.yml changes for review app
• Merge this MR
• Merge https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29683

Also: discuss with the team the guidelines for embedding videos and document them.