Merge branch '23665-build-sidebar-has-extra-space-on-firefox' into 'master'

Resolve "Build sidebar has extra space on Firefox"

## What does this MR do?

Accounts for the deprecated use of `document.body.scrollTop` on FF that was breaking the `translateSidebar` method.

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

## Why was this MR needed?

## Screenshots (if relevant)

![2016-10-22_23.16.31](/uploads/496810a9bb4fa4c3762023ff175572cb/2016-10-22_23.16.31.gif)

## Does this MR meet the acceptance criteria?

- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG.md) 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 it does - 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 #23665

See merge request !7060

CHANGELOG.md

@@ -5,6 +5,7 @@ Please view this file on the master branch, on stable branches it's out of date.
   - Trim leading and trailing whitespace on project_path (Linus Thiel)
   - Prevent award emoji via notes for issues/MRs authored by user (barthc)
   - Adds an optional path parameter to the Commits API to filter commits by path (Luis HGO)
+  - Fix extra space on Build sidebar on Firefox !7060
   - Fix HipChat notifications rendering (airatshigapov, eisnerd)
   - Add hover to trash icon in notes !7008 (blackst0ne)
   - Simpler arguments passed to named_route on toggle_award_url helper method

app/assets/javascripts/build.js

@@ -148,7 +148,7 @@
     };
 
     Build.prototype.translateSidebar = function(e) {
-      var newPosition = this.sidebarTranslationLimits.max - document.body.scrollTop;
+      var newPosition = this.sidebarTranslationLimits.max - (document.body.scrollTop || document.documentElement.scrollTop);
       if (newPosition < this.sidebarTranslationLimits.min) newPosition = this.sidebarTranslationLimits.min;
       this.$sidebar.css({
         top: newPosition