Improved build page scroll UX

## What does this MR do?

This MR smoothes the UX of the builds page by more effectively affixing the scroll step buttons.

It also ensures the scroll step buttons are always in view, even if the sidemenu is open.

It also moves the autoscroll button into the same container as the scroll buttons.

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

## Why was this MR needed?

The build scroll buttons are always in unpredictable places and are often hidden behind sidemenus.

## Screenshots (if relevant)

![2016-09-08_17.43.58](/uploads/49cb9ad5ef2764453afaa405af7111b2/2016-09-08_17.43.58.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
- [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 you do - 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?

Contributes #21832

See merge request !6270