Enable display of admonition icons in Asciidoc.

## What does this MR do?

Enables admonition icons to display in rendered asciidoc documents.

## Are there points in the code the reviewer needs to double check?

## Why was this MR needed?

Gitlab support of asciidoc needs a lot of improvements to make it useful in enterprise settings.

## Screenshots (if relevant)

![Screenshot_20161129_132250](/uploads/d280336000eb8b445d07d9bb2a2ed639/Screenshot_20161129_132250.png)

## Does this MR meet the acceptance criteria?

- [x] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

See merge request !7812