Add borders to images in issues, MRs, and Help.

## What does this MR do?

Adds a gray border around images in Issues, MRs, and Help pages.

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

That this doesn't effect other elements or pages that it shouldn't.

## Why was this MR needed?

It was hard to differentiate between the actual interface and screenshots of it.

## What are the relevant issue numbers?

Fixes #18578 

## Screenshots (if relevant)

### Issues/MRs

Before:

![Screen_Shot_2016-06-14_at_12.22.08_PM](/uploads/00a242a5b4a09b5308f02e6e498862a8/Screen_Shot_2016-06-14_at_12.22.08_PM.png)

After:

![Screen_Shot_2016-06-14_at_12.22.30_PM](/uploads/8fc9fc1c561d788b050e48373449771b/Screen_Shot_2016-06-14_at_12.22.30_PM.png)

![Screen_Shot_2016-06-14_at_12.22.39_PM](/uploads/36c91b2f2ce1eeb77f1646415baea14d/Screen_Shot_2016-06-14_at_12.22.39_PM.png)

### Help

Before:

![Screen_Shot_2016-06-14_at_11.58.54_AM](/uploads/7620491ca338064b872efb538b0bf4f3/Screen_Shot_2016-06-14_at_11.58.54_AM.png)

After:

![Screen_Shot_2016-06-14_at_11.58.57_AM](/uploads/d9fcfcac0f70dde4e69ff4263e369f65/Screen_Shot_2016-06-14_at_11.58.57_AM.png)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [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

See merge request !4655