Set webkit-overflow-scrolling to auto for children of body.

## What does this MR do?

Fixes weird tooltip layering behavior in iOS Safari. See screenshots and/or the original issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/22816

## Why was this MR needed?

Tooltips were cutoff in Safari.

## Screenshots (if relevant)

Before: 
![Screen_Shot_2016-10-18_at_7.13.11_PM](/uploads/5558d60b7369a9355f18d34dcc2c179e/Screen_Shot_2016-10-18_at_7.13.11_PM.png)


After:
![Screen_Shot_2016-10-18_at_7.13.40_PM](/uploads/ae07002f2f396f135b3078538d5c4ad6/Screen_Shot_2016-10-18_at_7.13.40_PM.png)

Also, as part of this fix, I removed applications of `-webkit-overflow-scrolling: auto` in two other places where they're no longer needed. One was the file-holder. I made sure I could reproduce the behavior this was intended to fix, and then made sure this MR still fixed it. 

Here's the errant behavior: 

![2016-10-18_19.00.22](/uploads/0bd3ee3bab44769dfce80c7edaac3248/2016-10-18_19.00.22.gif)

Here's what it looks like with this MR:

![2016-10-18_19.00.49](/uploads/e405ded5acdbbbe5e577222c11198691/2016-10-18_19.00.49.gif)

## 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)
 - [ ] 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 !6962