Improve build trace scroll controls

## What does this MR do?

The trace step buttons now correctly scroll to the top or bottom of the build trace, rather than jumping inconsistently and never full reaching the top due to the fixed position of the header/navigation area.

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

## Why was this MR needed?

Build trace UX

## Screenshots (if relevant)

![2016-09-09_20.04.32](/uploads/efee5145fb505f8c9ae2a4cf24d842d3/2016-09-09_20.04.32.gif)

## Does this MR meet the acceptance criteria?

- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) 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 you do - 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?

Contributes to #21832.

See merge request !6288