Image sizing

## What does this MR do?

Limits image height to fit the screen. The wrapping div is so the image is guaranteed to be a block element without the link area growing to be larger than the image itself.

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

Make sure this can't be done in a more performant or concise way with Banzai.

## Why was this MR needed?

Images were displayed at their full resolution, which made it difficult to read issues when the image height was greater than the viewport height (see #18861).

## What are the relevant issue numbers?

Fixes #18861.

## Screenshots (if relevant)

Before:

![Screen_Shot_2016-06-20_at_3.25.26_PM](/uploads/158424375ade95adcd337ccd34c48747/Screen_Shot_2016-06-20_at_3.25.26_PM.png)

After:

![Screen_Shot_2016-06-20_at_3.24.57_PM](/uploads/f1a3b5f6442e4e3b1067332a547fb1c8/Screen_Shot_2016-06-20_at_3.24.57_PM.png)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [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 you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

cc: @jschatz1 @dzaporozhets @rspeicher

See merge request !4810