Truncated long user lists in award emoji tooltips

## What does this MR do?
Truncates award emoji tooltips so that they only show 10 users maximum.

Further users are indicated by appending "and X more."

## Are there points in the code the reviewer needs to double check?
Is 10 too little, should it be raised?

My test cases rely on join() to build the expected output.
This feels a little iffy is it alright?

## Why was this MR needed?
Some issues have a large number of thumbs causing tooltips to be very large.

## What are the relevant issue numbers?
closes #18334, closes #19542

## Screenshots (if relevant)
##### Before
![Screenshot_from_2016-06-20_19-49-12](/uploads/d7a14dd87bb3da2acd7c0818de99852b/Screenshot_from_2016-06-20_19-49-12.png)

##### After
![Screenshot_from_2016-06-20_19-50-58](/uploads/f7f05c44594bfe8cec7dfd48802753a6/Screenshot_from_2016-06-20_19-50-58.png)

Truncation point modified for purposes of screenshot

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] API support 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)

See merge request !4780