Update snippets UI

## What does this MR do?

Updates snippets UI _(mainly mobile view)_ to new UI proposed in #18849.

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

Does anyone know the implementation difference between `app/views/snippets/show.html.haml` and `app/views/projects/snippets/show.html.haml`? I have only been updating the latter as it's the only one I can find implemented. I assume the former is rendered somewhere seeing as there is controllers for it too. But this doesn't seem very DRY from my current perspective.

## Why was this MR needed?

## Screenshots (if relevant)

![Screen_Shot_2016-09-06_at_14.20.10](/uploads/ac124a3b0896872ce851b9f56fbe339c/Screen_Shot_2016-09-06_at_14.20.10.png)

![Screen_Shot_2016-09-06_at_14.20.32](/uploads/8d3c7b1d1a5f562ceb6dc22bde695289/Screen_Shot_2016-09-06_at_14.20.32.png)

![Screen_Shot_2016-09-06_at_14.20.48](/uploads/6b1c31ece6d2400bbfc3c673d9e82d4b/Screen_Shot_2016-09-06_at_14.20.48.png)

## Does this MR meet the acceptance criteria?

- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry 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
- [ ] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

Closes #18849

See merge request !6210