Improve Ci Linter to highlight invalid syntax elements

After finishing CI config refactoring (https://gitlab.com/gitlab-org/gitlab-ce/issues/15060) it will become possible to improve CI Linter to highlight syntax of .gitlab-ci.yml.

We can make it possible to render syntax, highlight errors and provide a tooltip with description of an error when someone hovers a mouse above highlighted text. After that we can render .gitlab-ci.yml in the file browser using this new feature.

Adding this feature may make it easier for users to manage their .gitlab-ci.yml, especially when someone has a really complex CI setup.

/cc @markpundsack @ayufan